Isn't this fundamentally the same as asking why PayPal isn't a bank and Uber isn't a taxi company? I'm all for tech skepticism, but I think it's pretty naive to suggest that the platform is not a major part of a business at this point in the game.
The down-stream network effects of this are likely more significant and harder to predict, as real estate is way slower than hailing a taxi. Think desegregation.
As such, it is better for Redfin to operate in the markets where house prices are higher since the absolute amount of money they can save is also higher in these markets.
Brokerages also have network effects; they just end up being nullified by the MLS. If some real estate website somehow cornered the market on listings by dominating the seller side, they could have a significant network effect by either not publishing to the MLS or doing so after a delay.