Because you are not actually attracted to them. You like the chase. I had a friend that only liked guys that other girls liked. When she ended up dating them, she would break up with them shortly afterward.
Some people like the thrill of the hunt better than the actual catch. It sounds like you are one of those people, and realize that you have that problem (which is a good thing to realize).
Don't go after these guys. They won't make you happy. Wait until you find a guy that does enter your idle thoughts. You will know when you find someone you are truly attracted to.
There is no reason to feel the need to compete, because at the end of the day, it wont make you happy.