> Bookmakers can always lay the odds so that there’s equal money on each side

No they can't. Large ones can, but the small ones sometimes cannot because they have to match the large ones but in a smaller pool. If everyone knows which team is going to win, diehard fans of the losing team will still bet on their team - large players capture that in their odds and break even. Small players don't have the same mix of customers and so will have to take that loss because they need to compete with the large ones.

Of course overall the small players balance enough the 10% rake from less sure games over the years to make a lot of money but they can still lose 6 figures on some individual games. Small players are less a target for the pros (if only because they know their customer better and so won't accept the pros in the first place).

Remember the small players are often running an illegal operation. They need their reputation of paying out so that customers don't go elsewhere to even turn them in.

I wonder if street bookies make money “for someone” by somehow bringing lending in house. It would be the pair of activities that would be profitable. But they wouldn’t have to be that good at bookmaking.