Because, for the most part, retail investors are not well-informed investors and are willing to cross the bid-ask spread. To simplify dramatically, suppose Citadel thinks the fair value of a stock is 25.185, the market is 25.18-25.19, and a Robin Hood customer wants to pay 25.19 to buy 1 share of the stock. If they buy from Citadel, Citadel makes $0.0050 on average. Given those basic economics, it makes sense for Citadel to pay, say $0.0018, to make sure that the Robin Hood customer buys from them instead of from Knight.