Why does he still want my attention if he has a girlfriend?

This is a big no no. You don't want a guy that is "cheating" on his g/f with you. And he might not be cheating physically, but he's cheating emotionally. If he was a real man, he would break up with his girlfriend before he even thought about another woman. You don't want a guy like this. Because if he will "cheat" on his g/f with you, and leave his g/f for you, he will leave you for another girl. It's a lose/lose situation.

He wants to get to know you better. You obviously come off as interesting/intriguing/attractive to him, so I would say to make that time.

It's possible that he wants to see what other options he has. It's also possible that he wants to go out with you as a friend.

Honestly, I wouldn't trust him. If he is willing to cheat on his girlfriend, then he would do it to you. Maybe he wants a one-night stand, but surely you respect yourself more than that! If he is really interested in you, then he would leave his girlfriend alone and make honest time with you. Right now, he's being dishonest with his girlfriend which isn't fair at all.