Yea, my understanding is more about the differentiation of that particular sort of high-mids most of the time. Having headphones or such with clear imaging and a reasonable EQ that doesn't cut that portion of the spectrum can matter to some people, in much the way that OP was disappointed with how a vocal/instrument studio headphone pair sounded empty and weird.

Because it's probably the most sound-focused thing to check for? If you're playing any competitive FPS, it's a common sound you're looking for, and competitive FPSs are one of the most popular genres in the world. What's another gaming-specific sound that people care about hearing clearly that's not specific to a certain game?

In some ways, footsteps are essentially the "Vocal clarity" of gaming. Plenty of people listen to music without vocals, just like a lot of people play games where footsteps don't matter, but it's a nice marker that a lot of people care about.