Honestly I'd say it's less that Barbarian is frontloaded (it is but not to heavily). and more that Fighter is perhaps the single most frontloaded class in the entire game outside of debatably Hexblade Warlock.

Fighter is legitimately the only class that gets no actual dead levels until 9th (discounting the fact that Extra Attack doesn't stack) at which point the entire class immediately stops granting much of value for multiple levels, and the features you get in those first three levels are honestly far better than anything barring a Hexblade Warlock in a Charisma based build. literally anyone can take 2-4 levels of fighter and get a lot more out of it than any other choice.