the sell order flow to market makers who gobble up the other side of bad retail trades
I suspect they're in the business of collecting the spread on lots of small trades that they can assume are largely random.
But yes, the market maker doesn't run the risk of trading with someone with knowledge and a lot of capital to apply it.
NBBO requires that if there is something better that Robinhood gives it to you.
If you just hold your index fund, you don't pay these recurring spreads.