They add decisions, and decision making is the core of any game, way more in RPGs.
What decisions? There is no decision.
Why do you need STR to wield a sword? Because you have to decide between putting points in STR to wield better strength-based items or put points in other attributes.
given that STR increases your damage output AND carry capacity, there is already incentive enough to put points into it.
For a decision to be meaningful, both options have to be equally tempting. And they are not. Everyone and their dog makes fighters by pumping STR and mages by pumping INT because the entire game is practically herding you to build that kind of a character.
Does it make sense? No it doesn't! But so does not Weapons giving you points in attributes, like why holding a sword makes me stronger? And why holding a sword makes me even better at wielding swords?! (like a two-handed bonus)
That actually makes sense. Enchantment. Soul of a swordmaster bound to the sword? Etc..
However, why exactly stronger enchanted swords weigh more...that doesn't make sense. Enchantments don't have weight.
Are there alternatives? Yes there are, but when you have items that gives you bonus to important stats, it does make sense to require stats to wield them, because that way you can control the flow of the game.
There is a difference between requireing sensible stats and ever-increasing gateways.
Ever played D&D? Or Baldurs Gate?
Armors and swords have specific requirement, but those requirements are fixed. ALL two-handers rougly require 14 STR (IIRC.. alos D&D doesn't shower you in attribute points either and the range is limited so choices matter a lot), from the most simple blade to the Holy Avenger: Carsnomyr. And you could equip and wield that sword as soon as you got it with no penalties. Why? Because it's a sword, not a space shuttle.
Same for armor. You wear it. What does "level" matter? I can't wear this cap because I'm too low level? What, did I forget how to wear caps?
What a game shouldn't do is try to control the player and the flow too much. That's what movies do. They are linear experiences over which the directors has total control. Games shouldn't be that. Any needless restriction that isn't necessary is a point against the game.
