I don't understand this either and have never had an answer to that same question. Maybe it's because this game started life as a simcade affair and there's no facility to change the original code to allow tyre pressure adjustments.
Mainly it's because R3E has had a pretty weird development cycle. For a very long time, it's main use was in the Raceroom booths, and most of the time those are jump-in-and-drive. So a lot of options that most sim-racers take for granted were left out because they weren't deemed necessary at that time. Now that it has grown into a fully-fledged PC-sim, S3 find themselves with an engine that has a lot of things hard-coded into it Or rather, hard-coded out of it. Each patch gets us a bit closer, and I know that tire-pressures are coming at some point......you know, 'soon'. But trying to change the basic structure of a game, while also keeping Raceroom happy, and simultaneously trying to generate an income through new content, is a rather unenviable task.