This is a great question, and I echo many of the sentiments here and elsewhere around general complacency, slowing rate of change (even as bits of technology get better faster), and so on.
But...
One of the reasons that big projects used to happen so fast, is that the interests of people negatively affected by those projects were quickly and efficiently ignored. Of course that's great being able to look back at the big projects is inevitably good, but it's not so great if you were one of the people whose interests were ignored. An advantage of the stasis we seem to be stuck in now, is that we as a society are not quite so willing to stomp all over the interests of whoever happens to be in the way of a great new idea. Is this a good trade-off overall? That's not so clear, but there are good intentions on both sides of the "how fast should be do big things?" question.