Because a snapshot is only for protecting against minor mistakes and not a catastrophic error. A snapshot and a backup are not the same thing. Not even close. A snapshot resides almost always on the same system as you are changing and depends on the existence of the element/VM/whatever you’re altering as well as the continued viability and existence of the underlying infrastructure that it all collectively lives on.
Meaning that a snapshot is only useful if you do not destroy or otherwise render the greater system you are working on effectively inoperable, or do to the same to the target system that the snapshot depends on. If you do either of those and do not have a backup, you are gonna have a bad time.
And yes, I’ve seen it happen. Not to me personally, luckily, because I am paranoid about good backups. But I have helped many other IT people recover from catastrophic errors where they thought a snapshot was good enough.
It wasn’t.