Why do guys do this?

Some guys do that for that very reason, because for the next couple of days all you will think about is the guy, it'll totally drive you mad, and he'll be on your mind until you talk again. Sometimes with us guys it's an issue of finding more interesting conversation to have, if you've discussed everything under the sun to all extents, we don't want to seem like idiots because we have nothing more to say, and nothing is more annoying than sitting on the phone in silence. He could just genuinely be busy, what's his schedule like? does he have a job? is he a student? does he participate in many activities outside of either? are his hobbies very time consuming? there are many possible variables to consider.

boys are usually weird like that and they wouldn't talk to you about that stuff in public because it affects their"man-ness" and "ego". so just be happy with when you guys do talk closely on the phone or alone.=)