Why do girls LOVE to snoop thru my things when I am gone?

It could be your girlfriend sense that other girls are sending you signals, or other girls bluntly tell your girlfriend you're hot or cute or funny. There may be many other reasons, but it all goes back to one thing - insecurity. And it need not be the girl's fault. It could have been how you made her feel, unknowingly.

You need to give your girl a greater sense of security. And do not ever get upset with her over this as she's going to feel you've something to hide and this will add to her tendency to snoop even more. If you love her, you need to help her. Make her feel secure by telling her more about your stuff on your own accord, updating her on your own accord. Make her feel secure by not being "missing in action" or unreachable or lying or appear to be cheating or appear to have the possibility of cheating. Communication helps. Make her feel secure by telling her how much you love her, how different she is from other girls, how lucky you are etc. But don't do all these out of the blue if you're not normally the kind who would do all these. In such a case, it is better that you have a heart to heart talk with her before you do things that'll make her feel secure.
Guys may think this is tiresome, but this is why a lot of relationships fail when they don't have to.

Maybe it's a security issue. Something you said, or are not doing makes them insecure. Or maybe they are just immature.

When a girl starts to snoop, it may be because she believes or suspects that you are up to something (doesn't matter how based the belief is in the truth)

Just my $0. 02

Seriously, I have never snooped because I wouldn't want a guy going through all my crap. But I think its just curiosity. Just blatant curiosity. They wanna see everything. Maybe they are looking for something that they think will reveal another side of you. Who knows. Do you have live in girlfriend or something that does this? If this is something that REALLY gets under your skin then I wouldn't leave a chick alone in your place. When you leave then she needs to leave too.