Stable coins are not dollars or euros. They are something else. Something way riskier than holding plain dollars. That's why you are getting rewarded with that fat interest rate. It is not free money by any means. You are taking a risk and I must say, for the amount of risk you are taking 5% is nothing because stable coins can go "poof" any time and you better ROI your initial investment before they do otherwise you'll lose everything there.
You're right. The reason why I chose it is that my financial condition prevents me from taking any risks. As for this mining plan of coinbase, my USDT is unsecured, and I can choose to take them away at any time. It makes me feel safe. My friend told me that when you hold USDT below 5K, you will get 1.6% of minerals every day, 5K-30K will get 2-2.4%, 30K-80K will get 2.7-3%, and 80K-150K will get 3.5- 4%, 5% will be obtained above 150K. I am considering whether to increase my USDT to 150K in the near future.