Apple has a browser. Apple sits on the CSS steering committee.

The assertion is that the major browser owners (Apple, Microsoft, Google, Mozilla) don't want competition, so they keep CSS complicated.

All four of these companies are members of the CSS Working Group. So the ability to do this is within their power.

IMHO, a sad and small minded conspiracy theory

Also very likely true.

Corporate having interests that are not aligned with the interests of the majority and using their funds and power to influence the outcome to their liking is not a new concept

How does Mozilla benefit from keeping it complicated? Or Microsoft, for that matter? Neither of them have a mobile App Store, so are they just playing along for the hell of it?

Mozilla and Microsoft certainly think it's important to protect/increase browser share. How old are you? I can remember in the 90s, Microsoft thought controlling the web browser market was so important that they literally gave theirs away for free while their biggest competitor was selling their browser in Target, Walmart, etc. for I forget, like $50?

You control the browser, you control the ad experience, the use data, etc.

It's unclear to me what an App Store has to do with anything. (Microsoft does have an app store, by the way. https://apps.microsoft.com/home?hl=en-US&gl=US )

I think the much more likely (and obvious) conclusion for CSS's complexity is a lot more benign, it's simply a patchwork language that has had too many cooks for far too long. You simply can't have different corporate interests working within the same project without it becoming a mess. However, even if there was just one guy working on it, you'd still run into the issue of complexity eventually due to the issue of CSS being extremely easy to implement. There's no versioning required anywhere, so how are you supposed to update old standards? At this point there's no way to refactor old stuff (and thus make it significantly less complicated) without completely breaking everything on the web.