Does anyone invest in stablecoin? I have never heard although it's possible to get a little profit sometimes. However, people use stablecoin to fix their profit or stop their loss by converting their coin to stable. There's no other use of stablecoin then this.
Honestly I could not get what exactly OP is referring to. It name itself implies about stable then from where we can get "returns"? I guess people are getting misguided by coinmarketcap ranking. Because, coinmarketcap is their Bible and anything at top rank is being wrong interpreted as a safe-heaven opportunity for big ROI. This is the reason why people could not differentiate stable coins from rest of coins and still talking about ROI against stablecoins.
For trading it would be good but for investing its a big NO NO. You cannot be rich with stablecoins.
You can be rich if you buy and hold stablecoins in the downmarkets. For example, if you hold 1 BTC by the times of late November 2017 and selling it for 19,500 USDT and then you buy back bitcoins by the time of December 2018 for all 19500 USDT but BTC price by end of 2018 was less than $4,000. So, you will get 4+ BTC. Now you would have been richer by 4 times.
No volatility and all of the value will be stable, people will not do pump and dump
Only USD vale is stable but if you check stablecoin's BTC value will fluctuate along with BTC value so, it can be dumped and pumped against BTC. This kind of manipulations are happening on derived exchanges in more number compared to spot exchanges. So, we usually do not notice them.