Because they do not use same methods nor damage the map in permanent way.
There are 2 ways to do replays, what coh does with list of commands being executed in an order(and RNG outcomes playing as they did), or pretty much a 'video' kind of thing akin to SC2 replays.
Hmm couldn't you save states at say specific intervals (5mins, 1min, etc.) so you could get a way to rewind ?