This highlights the problem of averages. Most (99.87% or so) humans download zero npm packages. But those that do, often download them in the thousands at a time. And yes, clean-room CI servers are a big part of that.
You might be surprised (or maybe not) to learn that many service providers are far more willing to spend money on predictably large bandwidth bills than on less predictable changes in their infrastructure which require human time and attention to implement.
And perhaps the CI server folks would want this anyway because it would be vastly faster.