Because a lot of people depend on references from their previous job, including their managers and such, for the next one. Burning that bridge because you wouldn’t spend your last few weeks at the company doing what was asked of you is not a good look. It makes you appear difficult/like you hold grudges. It also might cost you things like your severance.

If it’s that important to you to flip a middle finger to your previous employer then go ahead, but I think most people will decide the cons vastly outweigh the pros. Especially since that person will get trained anyway so you can’t even meaningfully change things.