Why is a healer whose only personality is "If someone is injured, then I cast cure," less fun than a fighter whose only personality is "If it looks hostile, then I attack it"?
Why would a (hypothetical) healer that could only heal, be any worse of a design than a fighter that could only attack?
Because attacking is, quite simply, seen as more fun, more proactive and more consequential. And rightly so. Sure, there are some players who like being the healer, but they’re a lot less common than the players who like killing things.
As for bad designs, both would be bad, but the mono-healer would be worse.