The government regulations just say "the pillar must be this strong"

Not really. I don't think there exists a standard that directly dictates a-pillar length. In fact, they have to hire large teams of engineers to decide how thick it must be. It's a very challenging question - one that is solved via engineering and design and cannot be solved wholesale via a blanket regulation.

What the government does regulate is roof crushing resistance and a side-impact system that accommodates curtain airbags. The engineers have to balance these requirements with ensuring fitment for airbags, wire looms, and cushioning. The "a-pillar useless plastic aesthetics" being called out in this thread are far from useless. The a-pillar is one of the things a driver is most likely to bonk their head on. The engineers have to strategically place padding inside these pillars.