Why are BG3 martials so good

[LudicSavant]

BG3 includes heavy rebalancing of 5e courtesy of Larian, be it Fireball having a much tinier area of effect (it's closer in size to Shatter than the original 5e Fireball), or Tavern Brawler being far superior to GWM, or surfaces and various control effects no longer stacking, or high ground attack bonuses being incredibly easy to get, or jump giving everyone a bonus action move, or a completely new itemization system, or maneuvers on weapons, or a powerful alchemy system, or abundant powerful consumables, or thrown weapons becoming far better, or a lot of class features being added or changed, or quite a few spells being completely reworked or outright removed, or rewriting the initiative system, and so on and so forth.

It's not any one single thing. There's a lot of buffs and nerfs and changes that considerably shake up the balance of 5e.