And Tether doesn't have an unlimited supply of money. Tether allows USDT to be redeemed for USD. If Tether was minting unbacked USDT, then Tether would eventually be shown to be insolvent.
Second, if you're manipulating the price upward, it becomes more and more expensive to maintain the price. Like let's say Tether issued USDT to pump the price of BTC. If they wanted to maintain that long-term, they'd need to print more and more USDT to maintain the price, which gets us back to insolvency.
Third, the CEX would need a reason to pump the price. Usually, people pump prices to execute a "pump and dump" where low-value assets are dumped on unsuspecting consumers at a high price. It's possible, of course, but executing a pump and dump scheme is a lot of work and very risky for very little reward with such expensive, highly traded asset like BTC.