[Chaos]

Having too high BW reduces your movement, as well as hit/evade. There's also the fact that magic comes with elemental damage, that mages often have fewer skill slots than their melee counterparts, magic having to deal with Elemental Attack while most melee skills have that bonus tied into Scaled Weapon Attack bonuses, etc.

1. More reason to not dump your STR
2. So does numerous melee skills, a number of weapons, anything that utilizes Elemental Augment... pure casters are not the only ones in the 'my skills have elemental damage' department
3. Mages have a Fluency subtalent that adds up to five skill slots just for having a Tome, compared to the melee counterparts that have to dump around 25 points between SKI/GUI (or, god forbid, 50 points in WIL) just for the same increase in skill slots
4. Except you ignore the fact that each and every spell that bothers with elemental scaling also calls upon 100% scaled ATK, which usually comes from weapons that tie into the focused element(s). If you're looking purely at what the scalings give, most melee skills lag behind compared to offensive spells

Compared to the 'melee counterparts' you're suggesting, mages have it pretty good. Good enough to not worry about the weights of Books.