We are users like you and don't know why Apple set that policy. By having the addresses different, there are 2 email accounts that you can be contacted at instead of one. A backup if needed.
You should be able to log in below and change the notifications email. Even if you change the notifications email, you probably won't be able to use it as your Apple ID immediately. Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Reachable at or Rescue e-mail address, then it can't be reused with another Apple ID for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again. You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.
Apple ID - Manage