I can kind of see why people do it. Sometimes it's to hide the fact they have such low subscriber counts and that they want to focus on the content rather than the milestone. Sometimes people see a low subcount as an indicator of the quality of the video. No subcount shown means you have to trust and watch the video on it's own merit before you judge. Others think people won't take a channel with under 100 subs seriously, so they hide it to give themselves a better chance of being taken seriously when taking the focus off the nuber.
That said, there's also those who manipulate the system, but they can't manipulate the views. They think JUST having 1000 subscribers will mean getting monetised. They have such high sub counts, but their view counts are super low, so instead of being pulled up on why a 1000 subscriber channel can only get 10 views on a video, they instead hide the subcount in the hopes of not being noticed or called out.
So there's multiple reasons. Some valid, some suspicious.