Tether holds closer to 200B USD reserves (customer money) and keeps all interest on that, it's a ridiculously good business. Jeopardizing that in any way would be absurd.
Just assuming treasury returns they're making more than 7B USD profit a year with just a couple of employees.
You're assuming that the 200bn is all customer money. This is very doubtful to me considering that many times Tether magically starts minting billions of new coins to buy Bitcoin whenever Bitcoin prices start falling
You're also assuming that they make 7bn in profit per year just by keeping this rdiciulously large amount of money in a bank, but their own notation says that only 105bn is held as US treasury bills. The rest of the money is effectively invested, whether in gold or bitcoin or corporate bonds or loans. All subject to going down aswell as up.
Luckily for the cryptocurrency sector, Tether makes it so ridiculously hard to turn Tether into money that their bullshit probably won't be discovered for a long time, so the make believe valuations of bitcoin can stay as they are
The potential reward for putting it in something riskier is enormous. You and I might consider $7 billion/year to be plenty and there's no point in chasing higher returns, but we tend not to be the sort of person who is in the position to control a fifth of a trillion dollars in the first place. Investing it in something riskier to make more money would be foolish but it's also something that's been done many times before.
Just assuming treasury returns they're making more than 7B USD profit a year with just a couple of employees.