Why are bicycle tires incapable of maintaining pressure over time, while car tyres seem to have less of a problem?
A car tyre has much thicker rubber than a tubeless bike tyre, thicker still than a tube. The steel wires in the belts of a car tyre are impermeable to gas and occupy a significant fraction of the area of that layer.  So there is a material difference.  In both cases the rim can be regarded as impermeable, and the sidewall more permeable than the tread.

Car tyres, despite their much squarer cross-section, have a larger ratio of volume to surface area.  The volume determines the remaining pressure, while the surface area determines the leak rate.

Even taking that into account, car tyres don't really hold enough air for a year.  The fact that they may give acceptable handling for months on end has a lot to do with how forgiving driving a car can be compared to running on two wheels.  Try asking a motorcyclist (dimensions and construction close to car tyres) how often they top up the air.  But still, a few months should be OK on a car, though topping up once a month should improve your fuel consumption.

I don't run tubeless, so the following assumes tubes.  My tourer and hybrid, pumped to the max pressure, gives an acceptable ride for about 3 weeks on ~32mm tyres with cheap tubes.  That's starting from nearly 6 bar (85 psi) so over twice what most cars take.  My MTB doesn't get ridden much at the moment, but again with cheap tubes (though obviously fatter) can hold trail pressures for a little longer than that.  In practice I pump it up almost every use, because I tend to ride on road to the trails then let out some air.