The question must become "what is the other mechanism?". In the case of Tether, which has the largest market cap of any stablecoin, as far as I know, the scam is this. If I want to have liquidity on Bitfinex, I need to go buy tethers. If I try to make a transaction for bitcoins with dollars on the exchange, it could take days to process. There's no particular reason for this, You're just trading shares of a large wallet. So I must first go buy some tether from bitfinex, but wait, USD to Tether liquidity is also artifically capped. If I try to buy tether for dollars, it can also take days.
It follows that the only way to obtain liquidity on one of these exchanges is to buy and hold stablecoins. This is the premise upon which the exchanges that print these currencies can then turn around and lend the dollars used to buy them.
If you discovered, suddenly, that a casino had 78 billion dollars in chips just floating around, you would rightly wonder what the hell was going on, although the explanation would likely be different.
Tether is not a cryptocurrency, it's a casino chip. An escrow account that the bank is illegally investing in risky securities, entirely for the profit of its executives, investment in which is entirely funded by manipulating access to the wider crypto market.