Because attorneys know how deep the employer's pockets are, which gives them a great incentive to pull out all the stops, which gives corp HR the incentive to make sure there is a long and painful performance management process before anyone can be fired without it exposing the company to too much risk.
This doesn't apply to right-to-work states. Nuance is required in an argument like this. For nealy half the country, "hard to fire" is bullshit FUD. It's another excuse to be lazy in interviewing and follow FAANG in lock step.
Having lived and worked in a right to work state all my life, your comment does not ring true for my experience. Employees can be members of a "protected class" (one of which is simply being over age 40) which is often the core of an improper firing civil suit.
"right-to-work" means you can't be forced to join a union. It has nothing to do with this.
In other words, firing people in the US just costs money. Not very much, usually.