But its not? You can convert 50 million usd worth of bitcoin in one order without moving the market more than 1-1.5% and have a wire transfer to your bank account same day. This happens all day every day, with the markets getting deeper year by year.
Trash exchanges for retail have nothing to do with the infrastructure actually out there. If you are moving anything over 4 figures you shouldn't be trying to use retail exchanges. They're trash, slower, randomly flag stuff, and have higher fees.