There is some infrastructure (bridges, sewer systems, dams) that are supposed to last longer than that. Bridges are often torn down and replaced, and it's probably more expensive than spending twice as much and having it last five times as long (again, spitballed numbers), but that's what fits in budgets. They don't want to discover that traffic patterns have changed and they need a different bridge, or no bridge at all and have to take it down.
That's becoming a real problem for sewer systems, which in a lot of places are reaching expected lifespan, and it's going to be ludicrously expensive to replace.
Incidentally, there are also reports that the Brutalist buildings are so overbuilt that they're hard to get rid of, even when they're bad (such as having insufficient ventilation). Gigantic piles of concrete will be there in 2,000 years, whether we want them or not.
Taking the (shamefully low) minimum wage in the USA of $7.25/hr and assuming your employees work 40-hour weeks with 2 weeks vacation, that's $14,500 (again, shamefully low but let's roll with it).
Ten thousand of those workers costs you $145 million per year, and for twenty years that's some $2.9 Billion.
It's pretty obvious we could build a pyramid, which is basically just a hill of rubble with worked sides, for a lot less today if we only needed it to stay up for a few years.