It would be entirely reasonable for a magic glaive that gave the wielder Extra Attack, didn't require martial proficiency, and used your spell ability mod instead of STR to require attunement.
Which is far more than what the Bracers do. I mean, dang, a full extra attack? That's crazy good.
Using spells isn't something that is unique to one or two classes, either, yet items that allow non-magical classes to cast spells frequently require attunement. In fact, spells are one of the most common class features in the game.
They require attunement because of power. Being able to sling six fireballs a day is far far more powerful than +2 AC. That is six 3rd level slots of power.
Yes, class features entitle you to all kinds of things without attunement. That's why they're class features. I don't really understand what your argument here is. Are you arguing that if several classes can do something, a magic item that lets anybody do that shouldn't require attunement?
But, we just established that items that let you cast spells require attunement. Even for wizards and sorcerers. In fact, those items are generally restricted even further and only usable by those classes.
So, what about "I can wear armor" justifies getting a +2 AC for no attunement that can stack, while not wearing armor means that you have to have an unstackable attunement based item?
Yes, magic items that essentially give you access to class features you otherwise don't have often require attunement.
Okay, so if you're a monk, and you find +2 studded leather armor, wear it and see how having disadvantage on all attacks goes for you. Or, if you don't think attuning to Bracers of Defense is worthwhile, because you feel your 17 AC is more than enough fine, sell them for gold. Or give them to a beggar.
"If you think this item sucks, why don't you try ruining your class or throwing it away" I mean, it isn't like we can rewrite the item...
Oh wait. We can.
See, arguing that we should rewrite the rules of then game, generally is hard to counter by claiming that you can just play by the rules anyways. See, we don't use the Bracers of Defense. That's why I advocate for changing them, because I would like to use them, but they suck as written.