Stockbrokers are in the business of selling stocks to retail clients.
Their livelihood is from commission income, which in turn depends on their sales skills. This has nothing to do with trading. Desk and floor traders in brokerage firms are there to execute retail and institutional client orders. Most of them also trade for their own accounts and their firm's inventory accounts. Buying at the bid and then selling at the offer to execute all these client orders, and pocketing the spread, is very lucrative.