Different design, and last time I checked Gibson wasn't designing Fender Amps too. Gibson Had their own line of amps that didn't sound anything like the tweeds amps that were being produced by Fender. Leo Fender designed the Stratocaster with his Tweed amps in mind in fact you could get a Stratocaster shipped from the factory with a tweed champ as part of the package.

Well whateve the reason they first did it is irrelevant and pointless
the man's question was why they still do it, and thats because it tips a hat to the original it can't be functionality. I mean, even if you happen to only own one of these amps that needed access to the pickups full frequencies for it to sound right, they already make a Delta tone no-load pot. They continued to do it long after the selection of amps grew to huge numbers. Just like they still make bent steel saddles thatv don't have enough clearance for the stringb tob pass through to the saddle without having it press against the oval opening on there saddles.