Because they have to employ somebody at your position and it might as well be you. That they are indifferent between you and a market-rate replacement does not mean they don't benefit from having you in your position.

Say I'm in the market for a babysitter. If the market rate is, say, $18/hour, and I hire someone for $18/hour, I'm not abnormally benefiting from a low salary, yet I'm perfectly okay with that because I need a babysitter. The rate at which I no longer want to have a babysitter because the benefit they bring is cancelled out by their salary may be much higher than the market rate, because the market rate takes into account other potential babysitters.

In short, market rate is not the rate at which employers can no longer profitably hire, but simply an equilibrium price point that balances supply and demand. Assuming a reasonably well-run company, the price point at which they are better off not having you or anyone else in your position is much higher than your market price or worth.