You may not even notice it until it's almost flat, because it takes a keen eye to be able to tell by looking that a modern, short sidewall tire that is supposed to be inflated to 40 psi is only holding 22 psi.

Many people don't look – much less check – because they have been conditioned to rely on their car's tire pressure monitoring system to let them know (via a light in the gauge cluster) when a tire is running low. These tire pressure monitoring system have been federally required for many years now, in the wake of the  Ford Explorer/Firestone tire debacle, which you may remember. People drove their Explorers on under-inflated tires at high speeds; the resultant tire failure resulting in barrell-rolling Explorers and a number of injuries and deaths.

The tire pressure monitoring systems were supposed to correct the problem of people not checking their tire pressure and driving around on dangerously under-inflated tires (low air pressure increases friction which causes increased heat build-up and that can lead to tire failure; under-inflated tires also affect handling/braking performance).

The problem is the "check tires" warning light that's supposed to motivate people to check their tires' inflation pressure is regularly ignored, in part because the light not uncommonly comes on when the tire pressure is fine. After a while, people stop paying attention to it – even if it's on all the time.

But why are the tires losing air?

It may not be for the usual reasons, such as a nail in the tread. It is now common for sound-seeming tires to just slowly lose air for no apparent reason. It is not uncommon, when this happens, to have to manually check the tires every week or two – which people ought to be doing regardless – and fill the tires back up to the specified pressure. Which they ought to not have to do, unless there is a nail in the tread or some other kind of damage to the tire or the valve stem.