Because American drivers have normalized always driving 10 mph (16 km/h) over the speed limit.

Cops won't pull you over or write tickets if you're not at least 15 mph over, we basically don't have speed cameras, everyone's trying to win the rat race and dehumanizing other cars around them, and it's not considered morally wrong (by most) to break that specific part of the law.

So a single vehicle obeying the law will quickly get a long line of tailgaters and tailgaters of tailgaters trying to "push" the vehicle to go faster.

They can suck it, I'm not late or in a hurry, and my ancient truck, steel bumper, and class 5 receiver hitch will not be badly harmed by your plastic grille. I get better gas mileage and have a longer stopping distance when I drive the limit, and I don't care if others are honking or riding my ass because they think I should drive faster.

On most US highways (i.e. multi-lane limited access roads), it's customary to leave a path in the left 'passing lane' for any traffic that wants/needs to go faster than you. If cars match speeds across lanes, it impedes faster traffic.

The speed limit itself is a separate convention and regulation. In some places you can be cited for obstructing traffic by going the speed limit in the passing lane if you are matching the speed of cars to your right, effectively blocking the road.

> How do you get “stuck” behind someone doing the speed limit?

"Only 46.5 percent of U.S. drivers consider going more than 15 miles per hour over the speed limit on the freeway to be "extremely" or "very" dangerous — with 40.6 percent openly admitting to doing it at least "a few times" in the last 30 days" [1].

[1] https://usa.streetsblog.org/2023/11/30/why-so-many-u-s-drive...