Data caps are just documenting the reality that ISPs oversubscribe - if they sell a hundred 1Gb/s connections to a neighborhood, it's highly unlikely they're peering that neighborhood onto the Internet at large at 100Gb/s. I don't know what the current standard is, but in the past it's been 10/100 to 1 - so a hundred 1Gb/s connections might be sharing 1-10Gb/s of uplink; and if usage starts to saturate that they need a way of backing off that is "fair" - data caps are one of the ways they inform the customer of such.

I've seen it with my new fiber rollout - every single customer no matter their purchased speed had 1Gb up and down - as more customers came online and usage became higher, they're not limiting anyone, but you get closer to your advertised rate - but my upload is still faster than my download because most of my neighborhood is downloading, few are uploading.