For a lot of cases the answer is lobbying, e.g. in medical/healthcare there's practical 0 competition for a lot of products because of anti-competitive laws like really shitty intellectual property laws that let prices be controlled by few (collaborating) companies. Plus there's a lot of things that are technically illegal, but in practice laws only apply to the poors so they're not really illegal for corporations: https://en.wikipedia.org/wiki/Anti-competitive_practices#Types

quite a few of them are “natural monopolies”. for those unaware (source):

A natural monopoly is a type of monopoly in an industry or sector with high barriers to entry and start-up costs that prevent any rivals from competing. As such, a natural monopoly has only one efficient player. This company may be the only provider of a product or service in an industry or geographic location.

ie, cable companies, electricity suppliers, amazon. it’s really complicated and really expensive to build the infrastructure needed to meaningfully compete in those industries.

another relevant concept is the “network effect”, defined as (source):

a business principle that illustrates the idea that when more people use a product or service, its value increases.

this kind of thing is more applicable to things like social media companies (they’re more appealing the more users they have). this makes it hard to compete with social media companies because convincing people to use your new app is really hard if the usefulness of it depends on everyone’s friends already being on it. (this is also part of the reason twitter is taking so painfully long to die)

both concepts illustrate the different barriers to entry that exist when trying to compete with these giant companies. these barriers are also what allow these huge companies to get so complacent.

(i’m not happy about quoting investopedia or wharton, but they do give simple definitions of both concepts so i did it just this once.)

The Average Joe rarely has enough capital to start a promising enterprise and when they do manage to, they get bought up by the very same corporations they were trying to compete against.

We've reached a point where corpos can just buy out the competition and do whatever they please to it.

And it's not even a greed issue, really. The system is kill or be killed, a corporation that doesn't do this sort of shady thing won't make as much money and will be bought out by others that do.

We have to change the rules of the game.

Competition goes way down when all the different companies are owned by a very small set of huge companies. And for these companies it's easy to setup cartels and just simply not compete and jack up the prices. Competition happens when the companies must make a profit or die, not when conglomerates can trust that they're the only game in town.