A common reason people don't do it the way you're suggesting is dogma about code coverage.

If the code has branches for each possible case, and then an "else" to cover anything else (which raises an exception indicating something impossible has happened), there is often no good way to write a test that covers the "else" logic, since the coder doesn't believe there's a way to end up going there. Or if there is a way to force it to happen, it involves jumping through hoops that are too much effort.