(And no, I don't care about what this or that D&D book says about it, I mean strictly in the confines of BG3. BG3 has diverged enough that a 1:1 comparison is moot.)
Except, that is the main premise for it. You might not care, but it is a restriction that Larian has employed across a lot of the game.
BG3 is based on DnD 5e. Specifically, the Player Handbook (With a few reaches into other modules and some Larian Homebrew). Thus, you can only bind a single weapon and only melee weapons are viable to be bound (Binding ranged weapons is explicit to the Improved Pact Weapon Invocation from Xanthar's Guide to Everything).
As far as "Main hand only" that is probably a technical thing, due to them not having the system built for choosing a particular weapon slot (Or weapon in inventory) so it defaults to the main hand weapon. They could probably extend the system so it can pick any available weapon in a similar way to how the "Throw" action works... But for the most part that would be adding extra annoyance to most people as binding other items has little utility (Of course there are some exceptions, like if you want to bind an off-hand weapon because your mainhand already scales off CHR because it's Wyll's quest reward Rapier or Jahera's Scimitar)
Outside of that, there's the gameplay notion of you know... Not just having all your weapons scale off your spell casting stat. Like, Pact of the Blade has already been the strongest pact simply because of this feature to enable binding a single weapon. Binding everything and thus making everything CHR based would make it even more OP compared to the other Pacts.