TBF the only class that gets more than one extra attack is the fighter.

That's true, but other martial classes either get or already have features that increase the base damage of their attacks somehow, usually by or around 11th level, in order to roughly keep damage parity (granted, it's a stretch in actual play, but it's possible to see the workings - Barbarian has rage damage/reckless attack/brutal critical, monk has martial arts/fob/damage die increase to d8, Rogue has sneak attack, ranger has Favored Foe, paladin has smites, etc.)

The problem is, extra attack does not stack with itself, so a 5/5 martial has at least one dead level if both classes get it. That's not true of a 5/5 in two full spellcaster classes (excluding pact magic,) who in addition to getting higher circle spell slots from both classes, have access to 3rd level spells from each of those classes.

It bothers me a little because this was solved in 3.0 with BAB - you added the maximum bonus from each class, and you would get a number of attacks according to the multiples of 5.