Because to many...a sub-optimal choice isn't really a choice at all.
Now to be clear, there is a difference between weak "in the eye of the beholder" and weak "it just is".
For example, someone might argue that raising a Fighter's charisma over his strength is "sub-optimal". But the bottom line is, the fighter gets something for his charisma, and for his character that may be what matters. Optimized in this case depends on player preference.
However, when between two weapons, one is stronger, and the other offers no benefit at all....that is a problem. The only reason to take the weaker weapon is because it fits your character, but why should that force a player into an obviously bad choice?
At that point, I would just take the stronger weapon and reskin it...but then at that point why have different weapons at all?