Because technically speaking they are different things. "example.com." refers to a domain name in a system with a nameless root (like the Internet DNS namespace

Because technically speaking they are different things. "example.com." refers to a domain name in a system with a nameless root (like the Internet DNS namespace managed by ICANN). "example.com" would refer to some different (non existent) domain name system with a root called "com". The problem is that in practice many implementations that use DNS names display "example.com" to the user when they really mean "example.com." and many certificates are signed for "example.com" when they really mean to be signing for "example.com." A proper implementation would use "example.com." everywhere, but might choose to display "exmample.com" in the GUI if they consistently translate between the two.