Because everything is on Ethereum. Once you're in the Ethereum eco-system, there's no reason to leave. Once you're in Ethereum, then you can do whatever you want. Trade on DEXs, do swaps, lend, borrow, leverage, stablecoins, zk mixing, farm liquduity tokens, etc. Once you use Monero, if you ever want to do anything useful with your crypto, you're going to have to inject it into the Ethereum ecosystem, and that inherently means interacting with a CEX. So why not skip that step and just deposit your crypto into tornado.cash and wait for the pool to be sufficiently mixed. That way, you never need to interact with a CEX to actually use dapps on Ethereum.
That's true. It would be great to see some more variety in chains being used, but right now everything is being done on ethereum. We need some more bridges to other chains, otherwise there is no reason for anyone to spread out.