The small answer is that rendering touches not just CPU but storage as well. Yes we have SSDs, which help, but writing to "disk" is still relatively slow.
The big answer is: If you can render each of your pages very quickly, there's no real win from pre-rendering everything. You should just render on the fly. The whole point of pre-rendering, historically at least, is to make a site very fast by eating the cost of the render up front.
But from that storage "problem" you get to avoid hosting on a dynamic platform, and literally just need a static web host. As mentioned you can put stuff up on s3 and be done with it.
At large scale of visitors, this kind of approach is a lot easier to handle than the dynamic model.
The small answer is that rendering touches not just CPU but storage as well. Yes we have SSDs, which help, but writing to "disk" is still relatively slow.
The big answer is: If you can render each of your pages very quickly, there's no real win from pre-rendering everything. You should just render on the fly. The whole point of pre-rendering, historically at least, is to make a site very fast by eating the cost of the render up front.