There are certainly a lot of problems with incumbent ISPs that many other commenters have pointed out: deceptive marketing practices, anti-competitive behavior (ranging from lobby-driven legislation to lock out new entrants, to outright sabotage / destruction of cables), wildly asymmetric up/down speeds driven by economies of scale from 20 years ago, just to name a few.
Yes, even 4k video streaming at 20 Mbps is viable (assuming that your connection is stable). Yes, poor wifi performance / latency / bufferbloat / squirrels eating your coax are more common underlying causes of end-user pain points. But that doesn't mean that there aren't common use cases that benefit significantly from adding more bandwidth, nor does it mean that adding more upstream capacity is useless in the context of these other factors.
Also, let's be realistic: most regulators don't have anywhere close to the level of technical expertise that you'd expect from, say, a network engineer. It's a lot easier to reduce everything to the single dimension that is download speed, and it's a lot easier for a layperson to understand said changes.
It's also a straightforward way to place pressure on ISPs to upgrade their networks if they want to keep saying that X% of their customers have access to broadband plans.