The "make it part of the browser" argument doesn't work in practice because the GDPR covers the intent and purpose of data collection/processing rather than any specific technical way of collecting or processing said data. Blocking cookies at the browser level doesn't prevent the website from using browser fingerprinting or the information you manually provided (your delivery address to make a purchase for example) in a way you didn't agree with.

I’d really like a way for my browser to tell sites my default preferences, just to reduce browser noise.

I’d probably prefer more for the advertising industry to die a fast death, but I doubt that will ever happen.