Stadiums, airports, and semiconductor fabs. But yeah, we are off our game.
Space shuttle, reusable rockets, cell phone networks, Fibre deployment
They’re incremental (and out of order of importance), but these radically changed our lives in some way.
Airports can be complex, and of course chip fabs are an overwhelming case of quality over quantity, so in conclusion you're definitely right on the "hard" count still being accomplished, but I notice a particular gap missing here on "big".
Two generations is 40-60 years, so has the U.S. built anything big or hard since 1963? And I only have to name a single thing as an existence proof? I think I can! :-)
Apollo 11 landed on the moon in 1969. Since then, the Space Shuttle, Skylab, the ISS, various missions to Mars including as I recall, a helicopter. Hubble. James Webb Space Telescope. Does SpaceX building re-usable rockets count?
And for something more recent: how about Operation Warp Speed. Does not building a vaccine for a novel virus count?
Does the Boeing 747, introduced in 1970 count? How about the F-22? The MIM-104 Patriot?
Did you mean only infrastructure projects in the last 20 years, so since 2003? The Big Dig opened in 2003. Here's some other random projects I found completed since 2003:
https://en.wikipedia.org/wiki/Barclays_Center
https://en.wikipedia.org/wiki/Levi%27s_Stadium
https://en.wikipedia.org/wiki/Ohio_River_Bridges_Project
https://en.wikipedia.org/wiki/Evergreen_Point_Floating_Bridg...
Really complex things we completely fail at, Nuclear power plants take decades longer than intended. California's High Speed rail is billions of dollars over budget and many years behind schedule and may never finish.
The main issue we have in our built environment comes down to massively prioritizing automobile tech and privatized sprawl development at every turn, when we've known for some time that it's a money pit and doesn't produce the equitable outcomes promised. That's only shifted recently with changes in demographics(Boomers, the biggest block of homeowners, fading, immigration from places with different standards), and new direction from the higher levels of government(e.g. Biden admin infrastructure packages and policy-setting, or California's move to address housing by pressuring cities to build denser).
There's a definite correlation going on between "housing cost increase" and "infrastructure cost increase", which I believe has the causation mostly on the end of housing, and the trend towards financialization that set in at the end of the 70's and consolidated the homeowner demographic into voting towards asset inflation. The 2nd Ave subway was expensive because New York's land is expensive, not because the tunnel itself was a uniquely nefarious technical problem. But New York's land is expensive in a speculative sense: the structure of the tax policy and valuation is such that a rational move is to leave a building vacant and try to sell it for more, rather than set rent at a lower price and record a valuation in line with what tenants can afford.
Anyone "serious" about climate change realizes that it's a very slow moving thing that's not going to result in some sort of Hollywood Dante's Peak type scenario. Anyone "serious" about climate change also realizes it's neither fair nor realistic to expect up and coming continents like Africa to continue burning dung for heat while western nations get to reap the benefits of a century plus of fossil fuels turning them into superpowers.
Right now fossil fuels are still the best energy source you're going to get and still the best way of bringing nations to an industrialized point where they can actually consider "green energy".
Additionally, you seem to not understand that America has vast areas of emptiness and any federal plan for mass infra (especially transportation) is a massive waste of materials, taxes, and possibly incredibly destructive to certain eco-systems. Even my Dutch friends think it would be stupid to build mass transit on a European scale here. Our population is too spread out.
Lastly, stop thinking everything is a political issue. If you haven't figured it out by now... big government moves like a snail and will NEVER be the quickest or most efficient way to get anything done. If all this stuff is as good as you think it is, people will embrace it on their own without the need of any political battle.
Has the US successfully built anything big or hard in a generation or two? High speed rail? Subways? Tunnels? Mass transportation? Housing? Any infrastructure, really? We have a political problem and no amount of engineering or investment can fix it.
If the US were serious about climate change, I have no doubt we would move mountains tomorrow, but we absolutely are not serious. Sacrifice/compromise/responsibility aren't even part of the conversation.