Investing in stablecoins can be called investing only if your native currency is different from the dollar, and provided that your native currency depreciates faster than the dollar. But within the framework of a cryptocurrency, it cannot be called an investment if you buy stablecoins for dollars, you simply transfer one currency to another. Alas, the entire market depends on bitcoin, bitcoin, in turn, is volatile, which means that the whole market is volatile after it.
Investing into stable coins is actually a bad idea and I do store stable coins but that is when I feel the bitcoin market is going to go down and I need to convert my BTC into something solid. But investing into stablecoin doesn't make sense because you are investing into something that is meant to be stable so unless you get some staking rewards it's useless.
I believe stablecoins were made for the sole purpose to allow traders to have a basic currency which they can rely on. For example a trader can make all his trades throughout the day and at the end of the day he wants to ensure that the money he earned doesn't change so he can always convert his coins into stablecoins like USDT and relax.