Why do publishers agree to rights reversion? It reduces heir opportunity to directly profit and it increases competition for their other books they sell.
Most books (especially more than a year in) aren't actually a profit opportunity, and the effect of increasing competition for their other books is small enough to be totally negligible (at least in the current state of the market). And, as the other comment said, free ebooks can drive book sales.
It's often in the writer's contract that rights revert after a certain period, or if the book goes out of print, or if the publisher has not exercised the rights in the contract. Even if it isn't, a digital version might increase sales of a physical version, and some money is better than none.
