That's you opinion, not a cardinal rule. For example, Reddit comments and posts are deleted by users all the time.Same for Twitter, where users delete their posts and reacts all the time.
This asker ignored the rights of the answerers, moderators and SO, all of which put effort into creating that. I won't mind if someoen deletes his own unanswered question, but vandalising it after answerers put in an effort to answer? That's rude and egoistic.
If there is a delete button, it's fair game to use.
Yes, the lack of that changes how people participag on SO. I think it's a considerable reason why SO is the large resource that it is, with as many answers as it has. If askers could delete their questions (and thereby had the ability to make my answers un-google-able), I'm not sure I'd bother to answer, for example. Would you?
More to your point, it’s focused on evergreen information. Ideally the questions and answers get refined over time to resolve in to accurate documents that the public can use to solve problems. Most social media is about attention/dissemination in the moment and puts the individual above all.
When I say "rule", I'm speaking in terms of "how each person posting content should operate". Once you make your words public, you (should) have no expectation of being able to take them back.
It doesn't let you delete your question after someone has gone to the trouble of answering though. Good thing IMO.
I thought reddit keeps them still in the backend.