Most Helpful Girl
Maybe they're only attractive to you BECAUSE they have boyfriends. You see that they are desirable to men and therefore you want them too...
Also, you let yourself think of them that way.
When I know a guy has a girlfriend I would never ever think of them romantically/sexually. Like, they just go straight in the friendzone once I know and I would never think of them of anything more. You're obviously not doing that and still allowing yourself to admire them and think about them, they're unavailable to stop wasting your time :P
Most Helpful Guy
Because they're hot. They get hit on constantly.