The US is unusually lax about letting people leave its borders without any kind of controls in place. If you’re visiting on a visa, it’s actually on you to notify CBP when you leave through an online portal; they don’t even bother to track it via the passenger manifests (or maybe they do, but still insist on making you do the paperwork, like the IRS does with taxes?)
Most countries I’ve visited have a passport check on the way out as well as on the way in. Sometimes because not all of their residents are allowed to leave whenever they feel like it…
