Why does it mean if your ex boyfriend won't delete pictures of y'all from social media?

[MJ]

We'd been dating for about 3 months and he texted me and ended it, I deleted everything of him from my media and thought he'd do the same. He didnt so I asked him nicely to delete them and he said "why" and I explained how he ended the Relasonship, were not together anymore and that I didn't want my pictures on his wall anymore. He replied "just because we're not together anymore doesn't mean we have to forget" which confuses me because he ended it. And ended it with a crapy reason saying he didnt want a girlfriend but wanted to be friends, and all of our murtal friends say that reason is bull and says that he won't talk to anyone about it. Can someone explain why he won't delete our stuff if he ended it. ? and please don't say the reason is obvous without explaining it, he was my first boyfriend so I'm new to all the breakup stuff.