How tall are you? Have you actually tried any shorts on? I'm 5'10" and most bike shorts go down to the top of knee/bottom of my thigh muscles. I'd say that's pretty standard and I wouldn't consider it "short". If you're really concerned about covering up, consider going with knickers, which are mid-calf length. They may be a little warm in summer, but they are much more flattering than shorts. I do find that my Giordana Lazer shorts are a bit longer than any other shorts I have, but there's so much variance between brands and among different product lines that it's almost impossible to give you a clear answer.
Of course, your other option is MTB shorts...