My guess is that since the microphone is not enabled unless you are doing Voice recognition (a joke) or talking on the phone, that the system wasn't "listening" for noise. In fact, it really couldn't because if you have your music cranked, it couldn't tell if the loud noise was because of road noise or because of your music. (Unless you had a super smart computer that actually digitized your audio from the source and compared the background noise it was listening to with the audio source, filtered that out, and ... definitely not a "cheap" automobile solution.)
So ruling out a smart car "listening" to the noise in the cab, the only other option is what folks have eluded to with the MPH or RPM of the engine. In a normal car, these would make sense since the engine speed is a large contributor of noise. While we do hear an increase in road noise as we speed up, I'm not sure it's always uniform. Try this (I will): just have your radio off and go on a highway and speed on up and cruise making sure the car is putting a load on the ICE. Then try to find a flat or slightly downhill section and just coast at the same speed so the ICE turns off (or slows down). Is the noise in the cabin predictable? How about if I'm crawling up a mountain hill at 20 MPH but the engine is doing a billion RPM to keep me moving? So if the car decides the volume based off of RPM, then it won't turn up enough when the road noise at a high speed is the problem. And if it only ties to MPH, then when I'm crawling up the hill, the radio won't be loud enough to drown out the screaming engine...
My guess is the folks at Toyota found out that their volume adjust algorithm just couldn't take into account all the different factors that would contribute to inside noise, and that they couldn't reliably adjust the volume to make it appear uniform no matter what.
Me - I just crank it up and deal with it when I come to the stop lights. Never really had a self adjusting volume, so not missing much. My thumb is getting pretty good on the Vol. Up/Dn button. What I sure would like is a "Mute" button on the steering wheel for when I want to have instant silence! (Any suggestions... I could hit the Voice Recognition button (package #9), but that is temporary.)
