When you look back at how short men's shorts were in the 1970s and '80s, you have to ask why they didn't feel so self-conscious. Partly because people were more fit back then, including the legs -- not just man boobs and arm bulges from repetitive bench-pressing and curls. And their legs weren't over-developed from repetitive squats.
There was not a very big gym or "workout" culture back then, and yet the guys look non-freakishly fit. I've seen family pictures of my dad hanging out around age 30, and he had that look. So does Chevy Chase when they finally get to Wally World in Vacation, and so does Nick Nolte when scuba-diving in The Deep. If not the gym, how were they staying in shape?
There was a jogging craze around that time, but endurance runners don't get much muscle in the upper leg. In fact, they look pretty haggard. I remember my dad bike-riding a lot back then, although again if you've seen the typical schlub on a bike these days, you know that's not very plausible either. Not casual bike-riding anyway, another endurance sport.
You need some kind of explosive, intensive activity to develop decent leg muscle, and the average guy in the '80s wasn't training to be a football player or wrestler. Rather, what comes to mind is the mania back then for racquet sports that you just don't see anymore. In those sports, you aren't locomoting very much, kind of bouncing or staying prepared to sprint, but when you do move, it's in an intense burst. Every now and then, you're sprinting almost non-stop. And unlike joggers, sprinters develop larger legs. Plus it's not just running at a steady height, you're lunging down and springing back up.
Aside from stressing the upper leg muscles, all that burst-like movement gets your heart going a lot faster, above the threshold where your body recognizes that it's in a world where it's expected to really perform, and so it had better trim off the fat and toughen up the rest. If your activity level stays below that threshold, your body gets the signal that the world it has to interact with isn't so challenging -- just monotonous and never-ending. You improve your endurance, but not much else.
Unlike other sports, the equipment and clothing is pretty cheap for those played with rackets, and you don't need to get together a dozen others just to play a single game. And unlike working out at the gym, playing sports is social and fun. Both for overall psychological and physical fitness, it seems like racquet sports were just what the country was looking for.