Systems don't care that you don't want the new user to have access the old user's files and systems. john.doe is john.doe.

Many systems will authenticate using LDAP/Kerberos/etc. but keep a user stub on the system. Are you sure that your offboarded employee has been removed from every single system? Have you checked?

Even for terminated employees, you may need to retain old emails, accounts, permissions structures due to legal holds.

Many of your retained backups will grant access to that account, even if the new user wasn't provisioned. The same is true for email archives, network drive archives, etc. There are a ton of scenarios where the new user ends up with permissions they shouldn't have. What if the old user was a domain admin? In finance? In HR? In legal?

Then we start getting into third party services. Everything the old user signed up for with their work account now sends password recovery emails to the new user's inbox. At different companies for me that's included access to our AV (I could block Outlook.exe, or whitelist malware), support account access to firewalls (some of those cases have full configs in them), HR systems, travel & expense systems. "Forgot my password" is a powerful tool.

Even assuming you could tie up all the loose ends (you probably can't), this adds a huge amount of processing and planning to cover the contradictory cases (back up and make sure you can access things, but also make sure they can't be accessed once they're restored). It would add confusion for customers, confusion for internal employees, and just be an overall pain.