It's interesting to realize that one could be holding a phone and using a fraction of its computational capacity because the two companies that collaborated on creating it... don't talk enough?
It can also simply be a matter of available (human) resources. Remember how the iPhone was missing Copy & Paste? Probably not because they did not know how to do it. More likely simply a matter of shipping priorities and resources. I think the same is going on here.
While this is a good case for prioritization, I would argue that Google has made a mistake by allowing the fundamental interaction model of the phone to suffer from unnecessarily poor performance, either by taking less-than-full advantage of the GPU or (less likely?) not dealing with the problem of “heavy garbage collection” that is cited by the first project member to respond on the linked thread.
Copy and paste functionality is nice, but smooth-as-butter scrolling and near-instant responsiveness, a la iPhone, (or lack thereof) arguably define the experience.
Scrolling on an Evo or a Droid X is choppy, especially on a large webpage (zooming in and out as well). It's incredibly noticeable when comparing the most high-end Android devices next to an iPhone 4. If this isn't a priority, I don't know what is.
..or just don't care. Also, if the UI is not built with animation / OpenGL support built in from the ground up, it might be a substantial job to add good support for it later on. OSX did all of this from day one, so that making an iPhone without OpenGL support was probably out of the question. I personally feel that the 3D effects in Windows and Linux feel grafted on "just for the fun of it", and I usually turn most of them off. But I never used the iPhone and felt that I'd do without the 3D effects - it will even sometimes drop the 3D effects in favor of responsiveness when it needs to, which i think says a lot about how much work they put into it.
Hardware accelerated window composition wasn't introduced until Jaguar (http://en.wikipedia.org/wiki/Quartz_Compositor#Quartz_Extrem...), which was, admittedly, early on in OS X's life, and the first version that was usable for most people. 2D drawing is still done on the CPU (outside of Core Image).
