Because individual retail traders are (a) generally uninformed and (b) relative to institutions, have very small positions they want to move in the market. Those orders are intrinsically more valuable, because they aren't risky to make markets for.

Essentially, your trades have a lower cost basis than those of institutions. But the markets are generally priced assuming institutional traders. So if firms know you're not an exchange they can save some money on you, and Robinhood literally passes some of those savings to you.

(I don't love Robinhood or anything, but the model makes perfect sense.)