Because it takes a bit for the request to delete to get sent to every single instance that might have duplicated the post. It then takes time for the post to be deleted, assuming the instances that receive the request to delete follow the request. Also YOU yourself will still see the content because you will have the option to restore it if you didn't intend to delete it.

Same for when you post. I've noticed about an average of about 20 minutes before my posts are visible on other instances.

In the future, I would recommend assuming that anything you post to Lemmy or any other website/social media will be up there forever because once it's out in the open, it may be entirely impossible to remove every single instance of that post from every single thing that copied it. The server may be malicious itself and not honor any deletion requests, as well as users who may have seen the post archived it themselves.