As a web developer, sending an 8 KB JSON response is no problem. That's nice and light. In a networked action game, that's absurd. First, (hypothetical network programmer talking here) we're going to use UDP and write our own network layer on top of it to provide reliability and ordering for packets when we need it. We're going to define a compact binary format. Your character's position takes 96 bits in memory (float x, y, z); we'll start by reducing that to 18 bits per component, and we'll drop the z if you haven't jumped. Then we'll delta compress them vs the previous frame. Etc.
Really, what's happening is things are getting optimized as much as they need to be. If your game is running at 10 fps, it's going to get optimized. When it's hitting 60+ fps on all target platforms, developers stop optimizing, even if it could potentially be faster. Same for Reddit; it's fast enough for most users.