Because there is strictly speaking no time limit on how long US customs is allowed to detain non-citizens. IIRC the only limits are 2 days for you to be taken

Because there is strictly speaking no time limit on how long US customs is allowed to detain non-citizens. IIRC the only limits are 2 days for you to be taken to a facility (so you're not just being held in an interrogation room in an airport or wherever), and 90 days for you to be removed once there is an order for your deportation. The actual detention in between is limitless. Of course there's incentive to keep people moving through (and most people have a home country/embassy that they can eventually appeal to to expedite things), so in practice it isn't literally a de facto life sentence. These are things that have been happening at your borders (and, in different forms, at other borders around the world) for years - you're just less likely to see it if you have one of the strongest passports in the world. But to be fair, whenever you wake up is your own morning.