Sometimes it's because they have a reputation on the previous account and feel that a new account will be a chance to start over

But they don't make another account. They just keep posting as deleted

ahhh okay so that's what you're talking about
Sometimes people realize that they can still post/comment and do it for fun
Others do it bc they regretted deleting and still want to post/comment

I just find its kinda pointless because they won't get credited for it, not that that is what having an account is about, but they won't be acknowledged

They might actually get more acknowledgement from it. Surely you've seen people ask "who deleted"?

But there may be more then one person who has deleted their accounts

Yes,and they usually say who they were if you ask. Some even state their username when they leave comments

I just find it kinda pointless because you can't make it to the leaderboard or have people really recognize you

Some don't care about the leaderboard or recognition.