Because the order of cases is significant, these would be difficult to
work with in the REPL. If, for example, I start with your three
definitions, and then decide to write a special case¹ for 2, how do I
convince Python that
case def fib(2):
belongs before the general case?
¹ Yeah, I know, special cases aren't special enough. In a non-toy case
(pun intended), there could be any number of reasons to add a new case
in the middle, or to redfine/fix a case that already exists rather than
add a new case to the end.