How do they profit? There must be a cost somewhere? Another reply mentioned spreads - still a cost (you lose money when you trade).
Anyone who really cares about spreads will be using limit orders. Otherwise you're talking about pennies on highly liquid shares.
The fact that we're even discussing the possible spread differences between market makers shows just how commoditized retail trading has become.
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.
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.
They also earn some money from 'payment for order flow'.