We can build things when we want. However we have noticed that just building isn't always right. Urbanists often point to what Robert Moses did to New York City - he was able to build and he built a lot, but in order to build a lot he bulldozed a lot of the city which people object to, and many think he built the wrong things. So we put roadblocks in place to ensure there is more time to object should someone like him come along. Of course that means things we should build don't get built.

The other half of this is often we do not want. Many things that you think we should want to build are not important to the people in charge, and so they don't do anything. What matters to congress isn't that we build something it is that we use a lot of consultants and union jobs to build it - both those groups donate a lot to campaigns. (I call this corruption - you decide what you think). Note that congress doesn't care care what gets built in the end of it serves a useful purpose, just that we use a lot of consultants and union labor to build it. Blow up the costs by building a massive monument - this looks great at the ribbon cutting so congress loves it never mind that the costs mean we can't build very much.