What are you basing that on? How many people do you think are really using Google DNS or Cloudfare DNS? I think even if just Comcast changed their defaults, the root servers would see a order of magnitude more traffic than Google or Cloudfare see.

One of the reasons DNS is hierarchical is for load distribution (and performance). Taking this design principle and all of a sudden throwing it out the window would likely have major consequences for a system that was originally designed and scaled with it in mind.

For example, at the ISP I worked at for many years, we had ~30,000 customers. As an exmaple, Google specifies a SOa timeout of 10800 seconds (3 hours). Across the 4 resolving nameservers at the ISP, the most times they would request google.com in a day is (24 hours/day / 3 hours cached) * 4 servers. That's 42 requests a day, to service 30,000 people.

If all the customers were requesting it directly, it's a different story entirely. Say 80% of the customers access google regularly through email, or their phones, or whatever. Let's say they do it for an average of 8 hours a day (before work, after work, while sleeping and some random update/email comes in). In this case, the math is (8 hours/day / 3 hours cached) * (30,000 * 0.80). That's 64,000 requests a day to service 30,000 people (even though only 80% are actually visiting that domain).

That's over a thousand-fold increase, but it only gets worse if you look at larger ISPs, like Comcast and AT&T. Now, this is a pretty bad case, but there's plenty of very popular sites that are used by our devices on a regular basis, all day long. Doing direct requests from customer networks instead of using intermediary caching DNS servers is a horrible idea and would cripple the internet as the root servers fell over from massive load.