They could but they're saving it.
Dilithium was running low. 120 years ago all dilithium in active use went inert at once, causing every warp ship in the galaxy to have a warp core breach. Anything not in use during the burn continues to function and is very valuable.
OK. That makes no sense to me though.
So all they need to do is mine more to recover?
120+ years?
Or maybe it's a fear thing? No one wants to travel at warp because no one knows what caused the burn and they don't know if it will happen again?