I'm annoyed because my girlfriend sleeps about 70% of the time I see her.
Why does she sleep so often?
I dunno. She just likes falling asleep on my lap.
It seems rude that she sleeps so often when spending time with you. You should talk to her about it and see what the reason is.  Maybe she is tired from work.
I'm not sure. I won't do anything and she can sleep to her hearts content.
While that's very nice of you to be willing to let her relax and sleep, it's still a bit rude of her to spend so much of your time together sleeping. You said it annoys you, so there's no problem with you bringing it up and telling her that you wish she would stay awake more when you are around.