This is a common saying, and I’m sure there’s some truth to it. But IMHO, the main reason is that PIs just want to use the money.
Because… why not? It's free money. Having the newest shiny equipment, at the very least, boosts your group’s reputation.
Not to mention that straight-up corruption (pocketing funds for personal gain) is not unheard of.
That's not specific to the US, and it's because of a perverse incentive coming from those who assign the funds.
If you don't use them they cut your funding for the next cycle.