I’m guessing because a sidewall on the tire wouldn’t be “airless”? And they’d have to check/monitor the air pressure of each of the “pockets” of air as temperature changes?
Full disclosure: I say this as someone who knows next to nothing about tires/tire pressure/cars in general. My tire pressure light has been on for at least three months now.
"Under inflated tyres will increase stopping distances and affect your vehicle’s handling so that collisions are more likely; Under inflated tyres will flex more and get hotter, making sudden tyre failure – blowouts – more likely; Under or over inflation increases tyre wear so you’ll have to buy new tyres more often; Under inflation increases your tyre’s rolling resistance which means higher fuel consumption"
Indeed, I'm told it's a sensor malfunction not the tire pressure. Apparently it's a known issue with Toyotas. The mechanic is nice enough to check the pressure for me when he sees me getting gas since the "new" sensor (sensors?) didn't keep the light off very long.
Full disclosure: I say this as someone who knows next to nothing about tires/tire pressure/cars in general. My tire pressure light has been on for at least three months now.