Why can't git pull --rebase work if there are modified files?

If I have locally modified files and I try to 'git pull --rebase', it fails because of the locally modified files. If I don't use the --rebase option, it works (but generates a merge commit if I have local commits). Why does rebasing require an unmodified checkout? It seems like it should only stop if I something in the newly fetched changes conflict with my local changes.

I almost always have modified files in my checkout for things I'm working on. I also often have a commit or two that haven't been pushed because I'm waiting until I get to a good point before pushing. If I do 'git pull', I end up with a merge commit each time. That's why I want to use --rebase, but for it to work, I have to git stash, then rebase then git stash pop.