So the problem is in how volatile it is. Think of a term deposit or savings account as being the least volatile. Next year your money will have grown by 3% or whatever your interest rate is. Almost guaranteed.

Shares are much more volatile. Next year they might be worth 50% more. Or 5% more. But also could be worth 30% less, something that doesn't really happen with bank deposits.

Over a long period this averages out. You can normally expect something in the ballpark of 7-10% average annual return over a 10 year period for an index fund type one based on the S&P500.

But if you suddenly need the money, you might be forced to cash out when it's a -30% year.

There are cash-ish instruments like bonds that are often used in balanced investment funds, but the general idea is that you balance out the risk by having money split between the two kinds.

It's also a risk appetite thing. You might be a bit flexible so you might think you'll buy a house in 5 years, but happy to wait another year or two if the share market is down, then you might still be happy to put everything in shares.

If you need the money in 6 months or a year, you'll most likely want to put it in a term deposit that matches when you need it. There isn't a lot of gain to be made in that time.

If you'll need it in a few years, you might choose to split some into a term deposit and some into the S&P500 fund to balance the risk but also have a higher opportunity for growth.

There are funds that manage this for you. You want to make sure it's an index fund not actively managed (you can generally tell because the fees are vastly higher for actively managed funds), and you should be able to find funds that split between cash/income investments and stocks in different splits based on your risk appetite and timeframe.

I'm not in the US (and assume everyone on the internet is) so wouldn't be able to recommend anything specific. Where I live you'd expect fees to be less than say 0.5% of the invested amount. I understand the US should be less than this.

But if you see 1%-2% fees you're most likely looking at an actively managed fund, which you should avoid.