Nobody invests purely in a stablecoin if it's value is pegged to fiat. All else equal it's actually worse than holding dollars in a bank account which has a nominal interest rate.

The point of stable coins is to have a working capital system that is compatible with the rest of the crypto ecosystem. The reason is that most services in crypto aren't natively compatible with fiat due to fragmented legal & regulatory environments. Stablecoins are instant transfers to anywhere in the world. It's why a lot of countries are trying to build regulatory frameworks for stablecoins.

Two caveats: (1) USDC actually can earn a lot more in a coinbase earn account or vault than your dollars in your bank account, so lot's of people are saving with stable coins via higher interest rates -- I just wouldn't call it "investing".

(2) Some stable coins are pegged to a value that isn't real fiat money. Some good thoughts here: https://twitter.com/balajis/status/1422993084002934788