To explain some of my sickening tendencies: I'm not associated with any such for-profit hosting site, so I have nothing to gain from this. I just use them.
Implementing such a feature would cost resources that someone would have to pay for. Storage costs would go up, it's not atypical that e.g. a repo that's 100MB on disk might be 1.5x or 2x that (or beyond) if you were keeping every version of every ref ever. Think e.g. accumulating throwaway topic branches with library imports you never ended up using.
So how do you pay for running such a thing, nevermind the initial development cost?
You could just make it "free", but then you'd need to roll the cost onto customers across the board. Or you could only enable such "backups" for opt-in paying customers, but most people aren't going to think to enable/pay for that, or think "I won't need this", until they day they do.
So wouldn't it be neat to have such a service on in the background, funded by high premiums to recover the data in case their backup version is your last option?
I've certainly permanently lost personal data by accident where I'd wished I could have paid hundreds of dollars to get back, nevermind someone for whom such a thing might be of critical business importance.
Think about it as being able to pay money after-the-fact to undo the car crash you just got into. With technology that becomes feasible in some cases, and in particular due to how git stores data & what people tend to store there it's relatively cheap compared to some other types of storage.