As noted, if you had started the array, it would have said the disk was unmountable. That would have told you something was wrong. But of course, you already knew something was wrong.
How about this scenario instead. You put the disk into another Linux computer, and you actually write something to it. In that case, it will still mount, but it is out of sync with parity, and if you didn't know you had to rebuild parity, then you could have just continued on, and if you never corrected parity, then you wouldn't be able to rebuild another disk.
Short of a parity check, how can Unraid know you haven't done anything to any of your array disks when it was shutdown? And even then, it won't know why parity is out-of-sync. How could it?