Because often people break up with someone because they believe things will be better, less complicated, more available, If they are single. Often these plans don't play out straight away and they get paranoid, envious or suspicious that the person they dumped is in fact having a better time of it than they are.
Some people like to think they can go back if things don't work out. Obviously if the dumpee then goes out and gets a life the dumper then gets insecure, even annoyed that they might lose control over their ex.

Some people are just plain weird and even if they don't want you, they don't want anyone else having you either.

Mostly it about control. And not being able to understand that by leaving they have no fking say over your life anymore.
I see this b.s all the time with people who cheat on their spouse setting up with a new partner but refuse to divorce.
There are several reasons, none of them means he,actually has to give a damn though I am afraid.

All the best on shaking this one loose.