We have a "voluntary" tax system in the U.S. -- that's always been the situation. "Voluntary" doesn't mean that that you can choose to not volunteer to pay your taxes. It mostly just means that the way we run things, by default, it is each citizen's responsibility to calculate and pay their taxes each April.
American taxpayers filled out 1040 forms in the days before computers, a lot like they do now. The IRS selected certain fillings for audits, just like they do now -- sometimes because of an apparent discrepancy, and sometimes just at random.
It would be a lot more work, take a lot more resources, and be prone to a lot more error and lawsuits, if the IRS tried to calculate everyone's taxes for them. Even now that we are in the days of computers, it is much more efficient for the IRS to only audit a fraction of the filings submitted each year.
I'm also pretty sure our "voluntary" tax filling system has something to do with the Fourth Amendment and other privacy concerns. A lot of Americans very strongly believe that it is not the government's place to be all up in their private business.
-- EDIT to add:
There is a difference between whether it would be possible for the IRS to calculate individual citizens' taxes and whether we should abandon our voluntary tax system for one in which the IRS simply calculates the taxes owed by every citizen and send us each a bill. My original response was intended to address the latter, but now I'll say something about the former:
For someone whose single source of income is a job working for someone else, of course it is possible for the IRS to calculate your taxes. You've already volunteered all the information the IRS needs to do so. Your employer has already told the IRS exactly how much income you've earned and exactly how much of it you've had withheld for taxes. Remember when you signed that withholding paperwork with the HR department on your first day? That was the moment when you personally volunteered your income information and payments to the IRS. You've literally already been reporting your income and paying taxes on it ever since.
The way taxes work in practice for a single-income employee does not reveal the potential complexity of tax accounting for individuals who are self-employed, who have multiple sources of income, and anyone who doesn't want to make regular fillings and withholding payments throughout the year. The tax situation for single-income American employees is not the situation for all Americans. Not everyone has an employer who calculates their taxes and pays installments for them throughout the year.
It is common for Americans to have a single job with an employer who calculates and pays their taxes for them. This makes it very easy for the IRS to know exactly how much the taxpayer owes (or is owed) at the end of the year. If it ends up feeling to like this is the same thing as the IRS calculating your taxes for you, however, I'm guessing it's because you forgot that it's actually your employer who's been doing that accounting job for you all along, with each paycheck.
This case involved charges of fraud made against Trump's company by the State of New York. This was a civil case, not a criminal case. The consequences were not supposed to be criminal.
The defamation lawsuits brought by E. Jean Carroll were also civil cases. She was not charging Trump with the crime of raping her many years ago; She was suing him (twice) for lying about whether he raped her many years ago. (She won both times.)
I think I get where you are coming from, though. When a person is rich enough to pay the fine, and also shameless enough to revel in the infamy of being found liable in a civil dispute, it can seem like that person doesn't end up suffering any significant consequence for their actions at all.
$355M is a lot of money. Add in the $83M owed to Carroll and these recent fines top $400M, which is an estimated amount of Trump's liquid assets. Trump is now likely running out of cash-on-hand, which could explain his recent takeover of the Republican National Committee -- the GOP's fundraising (and fund-spending) organization.
Criminal consequences come from criminal cases. Trump has invested most of his legal defense against the criminal cases he is facing. Pending criminal cases involving Trump include:
1.) A RICO ("Racketeer Influenced and Corrupt Organizations") case charged by the State of Georgia, against Trump and several others who allegedly conspired to steal the state's 16 electoral votes, including by having the President call (Republican) Secretary of State Brad Raffensperger and ask him to "find 11,780 votes" for him. Four people in that case have already accepted a plea deal. This case is currently delayed by a motion to disqualify the DA because she had a romantic relationship with a lawyer her office hired to help prosecute the case.
2.) A federal case against Trump for retaining classified documents. A year or so ago, it was found that former President Trump and former VP Mike Pence had kept classified documents after they left office, and that when Joe Biden left the office of VP in 2017, he also kept some classified documents. Both Pence and Biden complied with federal investigation and surrendered the documents immediately when asked. Unlike Pence and Biden, Trump did not comply with federal investigation, and instead took action to conceal the classified documents in his possession. This case is being heard in a Florida courtroom, because Trump was storing these stolen national secrets in a spare bathroom at Mar-A-Lago. The judge is a Trump appointee, and has demonstrated a tendency to rule in Trumps favor whenever she can, but if she shows too much bias she may get taken off the case.
3.) A federal case against Trump for his involvement in the insurrectionist attempt to disrupt the electoral vote count in congress on January 6, 2021. Trump has been indicted on four charges in this case: "conspiracy to defraud the United States; conspiracy to obstruct an official proceeding; obstruction of and attempt to obstruct an official proceeding; and conspiracy against rights." Trump's defense has been that he has "absolute immunity" for any actions he took while serving as President. This claim of immunity has been denied and appealed multiple times. Trump has now asked the SCOTUS to hear his appeal, but they haven't said if they will yet. Until they do, that case is on hold, but there's no one else to appeal to higher than them. If SCOTUS chooses not to hear Trump's immunity appeal, the lower court's denial of it will stand and the case will go forward.