I think it doesn’t default because it keeps the same setup on the car even going from track to track. That way it only changes when you change it and keeps you from any surprises when showing up to a race. They probably don’t default to baseline when a new tire model is released in case someone hasn’t saved their current altered setup and don’t want to lose it for whatever reason (even though it’s probably useless with a NTM).
But anywho, when you get into a session (can be test/practice/qualify/race, whatever), there’s a tab up to the top right that says “garage.” Click that, then click “iracing setups” which will be a tab on the right side of the box. There should be one called “baseline.” Select it.
Any time they update the tire model, it’s a good idea to go back and reload the baseline, even if you have a custom setup. They recommend starting from scratch if you have your own tweaks you like to do since the car behavior may change a bit.