I have seen questions that ask "how can I X, without Y?" After accumulating a few answers they edit saying "oh wait sorry I meant with Y". There is assorted yelling in the comments and the person deletes the question entirely and starts fresh. This can happen in as little as 5 minutes. In that scenario, how would it help the internet, or even you, to insist they can't delete their "hot mess" and post something closer to what they meant? You might not even be answering the real question.
Other times, people realize the issue is not what they thought it was, but just a silly typo (missing a semi colon for example) or thought error (oh, right, I'm writing C#, not C++) and when it's pointed out to them they delete it, either out of embarrassment or because it really isn't going to help anyone for posterity to leave it around. Again, how does it help the internet, or you, to let you get a kick at answering that one just because you started to type?
And thirdly, sometimes people post spam or offensive crap, and moderators delete it. If the OP or a friend could roadblock that deletion by something as simple as starting to type an answer, that would be bad. Sure, we could say "lock it for OP deletion but not for mod deletion" and there's probably some sort of exception to that too. This just all leads to complicated code and UI for little or no benefit.
FGITW doesn't happen in every tag. If you're going to play FGITW, you have to play it as-is, with other people answering too, edits underneath you, stuff getting deleted and migrated, and all that jazz. Or, don't - restrict yourself to questions a few hours old that have probably settled down. Your choice.