Even ignoring technical considerations (the dizzying amount of code and cruft required to run a watch), it goes against one time-honored watch tradition: simple, elegant mechanics.
Congratulations. You've won today's "Find something to moan about" prize!
Seriously though - stop getting hung up on the word 'watch' - it's not the real point here. Nobody is arguing that telling the time doesn't require a full blown OS.
Even ignoring technical considerations (the dizzying amount of code and cruft required to run a watch), it goes against one time-honored watch tradition: simple, elegant mechanics.