I learned the fundamentals of graphic design through print design, as that is a medium that designers have worked with for much longer than the computer screen. A lot of designers approach print design by starting with a grid that describes how their content will be laid out, and proceed from there. Of course, you can find exceptions to this (although in many cases they will be an example of a designer breaking the rules because they know them so well), but most design students will be familiar with that approach.
"Grid Systems in Graphic Design" (1981) is one of the reference texts on this subject, if you're interested in getting deeper.
The other commenters say, in reply to your question, that it was just for formatting HTML, and for web documents rather than applications. That is just completely missing the point - print documents are documents too, not applications. The main argument that the web is so different from print because the canvas is infinite doesn't make much sense either - even if the canvas is conceptually infinite, it still renders on a finite surface (your screen), and one can design modular grid systems that easily accommodate that (as well as the other related concerns, like the user not having the right font installed, or different pixel densities/aspect ratios, etc.).
In the end, the best explanation seems to be that the CSS spec was never led by actual designers who could envision the sort of problems that one would face when designing documents for the web. The CSS spec committee is full of very hard working, smart people, but they have the concerns of engineers first - browser compatibility, rendering complexity, etc. - rather than designers.
The fact that the "holy grail of web design"[0] is a thing is absolutely mind blowing to me. This is the kind of design any first semester design student would execute, and any styling language that does not let you implement it in 10 lines is a styling language resolutely not designed by (or for) designers.