There is no real reason other than it conflicts with the rules about magic slots.
Why give power away for free? You're just going to unbalance the system for... what?
It seems to me that it is the current rule that makes the system unbalanced. Please correct me if I'm mistaken. As I said, there's currently no problem with having a second weapon instead of a shield.
Okay, take a couple of breaths and think about it, why can characters only wear two magic rings, why can characters not wear a magic cloak and a magic amulet or necklace? Because that's just how the magic item slot system works.It seems to me that it is the current rule that makes the system unbalanced. Please correct me if I'm mistaken. As I said, there's currently no problem with having a second weapon instead of a shield.
Already now, it is quite possible to make a very powerful two-handed weapon wielder, so I don't think this statement holds water. Using a shield or a two-handed weapon is a choice which determines the type of things you're good at, but one choice is not in absolute sense any more powerful than the other. We'll almost certainly see a barbarian-style class sometime within reasonable time, which will make such a style even easier to play optimally - but it's already the case that a shield fighter cannot approach the damage of a two-handed weapon fighter, nor make use of the reach which a two-hander can easily avail himself of. And a two-hander can do both, since switching weapons isn't hard, esp. with quickdraw.if you want a balance reason, it's because sword and board is already the best choice for those with proficiency, giving them an extra slot is buffing what is already the best option.
But there is no real advantage to dual wielding magical weapons either right ?
Barring special ranger powers (that I know admittingly little about) you cannot attack and benefit from both weapons at once.
All attacks is resolved from the main hand weapon, with the off-hand only providing some small bonuses if you have the proper feats. Even if it's uber magical.