The problem is controlling humidity. At least 99% of air follows ideal gas law at the pressures and temperatures we are discussing for tires, so it's that 1% or less of water vapor in air that can cause uncertainty with tire pressure as a function of tire temperature.
Nitrogen could be better in theory, but in practice (real world) air is good enough for my needs. I only add air a few times a year and with a small compressor in garage it only takes a minute. Since it works good enough why go to trouble and expense? My time isn't that valuable any more.
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