If I had to guess, I'd say it is your personality. I have a buddy who's really in shape and really good looking and bombs with girls, even with me right next to him running epic game on whole crowds and trying to pump him up. Do you have social anxiety? Do you get tense and awkward around girls? Do you try too hard to be "alpha" or "firm", and have a ton of pick-up related concepts in your head that you over do or poorly execute so that you come across as rude, socially awkward and unfriendly? How is your sense of humor, can you walk into a crowd and have them all cracking up?
Try RELAXING around girls, don't even worry about how good or bad your game is, just enjoy the moment and spending time socializing. See if you can be around new people vibing with them and making jokes. Are you easily able to make new friends when you meet new people? If you're only comfortable around old friends, you are probably wound up too tight and scaring girls off. You might need to work on your social skills, rather than game.
As for easily getting girls, imagine there are some number of categories that count. You have to have the whole package and can't be way below average on any one thing. Guys who find it easy have just gotten really good at most of those things, to the point where it becomes a snowball effect, each success making the next one even easier. Since human nature is to compare people's situations from what you yourself would do in them, it can become hard to relate, when you just fundamentally ARE really good with women. Put another way, just like other skills in life, it's really really hard, until suddenly it's just not and you can't figure out why it ever was, because you just GET it.