Uniswap DEX hit $50 billion USD in liquidity recently, mainly because it lets average people provide even a little liquidity into the trading pools and earn fees in return. Coinbase just declared $1.8 billion in revenue from trading fees in the first quarter 2021. On Uniswap those fees mostly goes to the liquidity providers so it is a powerful motivation to provide liquidity. Note that It is not risk free money if the asset trends in one direction for any length of time you can lose money vs just buying and holding.