These "free" stock trading companies cut deals with high-frequency trading firms that buy order flow. The more order flow they buy the more money they make (essentially risk free to them), which is why they are willing to send boatloads of money to companies like Alpaca/Robinhood.
High-frequency trading firms aren't in the business of stocks, they are in the business of building the fastest tech on the planet to connect buyers with sellers, therefore making markets more efficient.
https://www.kalzumeus.com/2019/6/26/how-brokerages-make-mone...