Right, but then you have the questions of 1) why do leptons have (a multiple of) the same fundamental unit as quarks, and 2) why does that multiple equal the number of quarks in a baryon, so that protons have a charge of exactly the same magnitude as electrons?

I mean, I guess you could say that charge comes from (or is) the coupling of the quark/lepton field to the electromagnetic field, and therefore if it's something that's quantized on the electromagnetic side of that, then quarks and leptons would have the same scale. I'm not sure that's the real answer, much less that it's proven. (But it might be - it's a long time since my physics degree...)