I'm sure it didn't spring up one day from nothing. The literal TGing was as I describe, pickup trucks with a cooler and sandwiches, really not much of a thing. I don't know when the tents started to happen. We can probably estimate when TVs started to happen because they'd have to be flat panel or little 13" battery powered jobs earlier than that. I'm not lugging some 32 inch tube set very far at all.
One can imagine as pickup trucks got fancier, the better off fans bought nice ones and could hook up fans and small TVs and bring a small grill etc., then a foldable tent, and viola.
I was pondering if it's old enough to be a tradition, but it's so endemic now, I'd say yes.
No, they definitely evolved slowly, organically, over time. As new technologies enabled us to do different things, we updated and evolved the experience.
We actually DID bring the 32" tube TVs, before flat panels were ubiquitous. They were huge and heavy and impractical and didn't come with non-glare screens so they were extremely difficult to see during daylight hours. But we did it anyway. Originally we used OTA signals, but then when the prices on Dish/DirecTV came down enough, we switched over to that and opened up a much larger list of channel options. We obviously had to have generators to run all of that, and so we also experimented with running "Cool Zone" type fans but they really didn't do enough to justify the hassle.
Eventually flat panel TVs came out and were lighter and easier to maneuver. When the LCDs became prevalent, they were also low enough power that we could run them off marine batteries with inverters and skip the generators. But once we got back up to running 3 TVs and various party lights and stuff, we brought back the generators.
I also mentioned the evolution of the shade structures-- we went from cheap tarps staked into the asphalt, to the large but unwieldy 10x20 temporary garages, and finally to the omnipresent 10x10 EZ-Ups.
Tons of other things we tried and evolved over the years. I've thought about writing a book about it, to be honest. Would be fun traveling to other places to see how tailgating evolved elsewhere. But from what I see on TV, it all looks about the same nowadays.