Why is easy. If your time machine migration does a horrible job then your only option is to do factory reset to get rid of inconsistency. That is why!Why would anyone ever need to restore or do a clean installation in a less than one week old machine? Apple even haven’t released any major OS update yet nor the prior version of OS works.
Furthermore it ia not logical to question users performing a common action that normally should not create the issue such as bricking a machine.