Why does my ex want me as a friend so desperately?

We had a super serious relationship for nearly two years, and then he broke up with me. But afterward he said he really wanted to be friends even though he knew I was struggling. He told me he wanted me to be like some other girl that he talked to about just school. I felt like I'd just be there for his entertainment. How can you be such casual friends with someone that you made commitments to? How do you even ask that in the first place?

How are you supposed to be friends with someone if you don't trust them? Like right before he left he wouldn't tell me specifics like people's names or where or what he was doing. He knew I wouldn't do anything about it, but he was just witholding information needlessly.

He was one of those kinds of people that never asked for something when they wanted it. So I was really surprised that he asked me for something like this. A normal thing for him would've been if he just broke up with me and left it at that. But he asked me to be his friend and he said he didn't want to lose me as one.

Additionally, he said that I didn't understand him so that just confuses me more. Why have a friend that will only frustrate you because they don't understand you?
Being friends with me seems like such a masochistic thing to do. I can see it in his face that every time he looks at me he's hurt. I don't know why he wants to keep dealing with that.