Because it is broken. Merges and rebases rarely work without manual intervention, a patch-oriented tool that is almost as fast as a snapshot-oriented tool, git. Something like Pijul looks promising.
It doesn't work because your changes conflict? How would anything else handle out of order modifications to files that cannot be resolved automatically? There will always been a requirement to do this manually - otherwise the VCS would be able to program for you (understand what is right/wrong).