Is there any reason I shouldn't delete orphaned database users?
As pointed out in a comment by eckes you should contact management and stakeholders for approval before deleting them.

Additionally if there are test or development instances that have copies of the DB, there may be logins on those instances where the users have access. You will want to consider impact there before making changes.

Part of reaching out to stakeholders is identifying who is currently the responsible business user(s).  It is not unusual to find a database that everyone forgot about and reaching out to these people that used to have access can be helpful.