I completely understand your position @Adlago. You are a power user with a very focused goal of attaining the highest possible pagespeed/CWV/lighthouse scores possible. This means spending days and weeks optimising CSS, JS, etc often making large amounts of changes and sacrifices to achieve your goal of a good rating from a brief audit with an automated tool.
That is what you want to do, and I understand that.
From my position, we want to deliver a very broad community platform that is capable of being used by a vast and diverse array of communities.
For example, these are both Invision Communities running the same major version.
Both of these websites use our base CSS. Now, if you wanted to make the Squarespace one super fast, you'd go in and remove virtually anything that is a colour, remove loads of the ipsBadge classes you won't need, all the various type styles and so on and save hundreds of bytes in the process. However, you could not then take that optimised CSS and make it work for the second example which has different needs.
My point here is that we cannot really create a very optimised set of CSS files for every potential community. What we want to do is overhaul the CSS to produce a much smaller footprint and be much more extensible but we don't have a magic wand. We have a finite number of developers with a growing list of things to do. Overhauling CSS means essentially starting a new theme from scratch and that means booking out our UI team for months.
Project planning is tough, but it's on the list to do.