They could, but as it currently stands media hosting on the fediverse.... Sucks.

It's obscenely expensive for everyone involved, and scales poorly. It's just not ready to operate at scale at this point.

I'm sure it will get better, but large storage costs are better off being handled by a distributed file-system where a minimal level of duplication is baked in, but the storage load is reasonably spread out instead of fully duplicated on each peer.

There are technologies for this, but they all have their own issues. And tomorrow there will be n+1 distributed filesystems, fragmenting it further.