ORD is in the middle of a multi-decade improvement project.

Runway additions have been completed which increased the airports capacity to at, or very near, the highest in the world. Taxiway reconfiguration is ongoing to allow for the Terminal 1 expansion which will, in turn, allow Terminal 2 to be completely rebuilt to include an international arrivals facility. Terminal 5 has just been expanded, with the new area being very modern, and renovation is proceeding on the existing areas. The extra gates added to T5 has allowed Southwest to be added, and Delta and Spirit(?) to move to T5 to make room for the work on T1 and T2.

LGA's new main terminal is beautiful and modern. All of the low-ceiling concourses with leaky roofs are gone. The project to connect all of the terminals continues.

EWR is about to open a brand new Terminal A. When it does, the old Terminal A will be decommissioned.

MCI has built a brand new terminal which should be opening soon. It will replace the circular concourses that were designed for a time when security was handled very differently.

AUS has extended their relatively new terminal.

SAT and MSY have new, modern terminals.

SLC is building a new terminal. Not sure if that's done yet.

IAH has a new north C concourse and work is in progress to replace and upgrade the D and E terminal.

BNA is in the middle of a very large terminal redesign and expansion project.

IAD was just (finally) connected to the Metro train system and is moving forward with a new concourse south of the existing C/D concourse that will eventually grow into the long-awaited replacement of that "temporary" concourse.

PHX just replaced Terminal 2 with an upgraded and expanded Terminal 3.

DEN is finishing up a project which significantly expanded the A, B, and C concourses adding more modern gates. Work continues on upgrading the existing areas.

SFO has recently opened a large expansion and upgrade to their southern concourses (don't remember the designation).

DFW recently remodeled the E terminal.

SAN is replacing Terminal 1. Terminal 2 is already quite modern.

I think these projects move slower than they should in the US because the government airport authorities, that operate most of our airports, move at the speed of government, which is very slowly.