>6.6.4 Inspiration from other formatting models In the early design phases of the CSS visual formatting model, other formatting models were frequently consulted for inspiration. In particular, TeX [Knuth 1984] was often brought up in white-board discussions.

As one of its foundations, TeX has a well-defined box model wherein all objects, including individual glyphs, are contained in boxes. The spacing between the boxes can be controlled through TeX commands. In addition to optimal spacing between boxes, TeX also allows maximum and minimum spacing to be expressed. This is referred to as glue (although Knuth suggests that springs is a better term [Knuth&Plass 1981]).

The visual formatting model in CSS is based on a box model, and all elements, both block-level and inline, are turned into boxes. Thus, CSS goes further than most other style sheet languages in creating boxes. For example, DSSSL and P94 do not use boxes for inline elements. However, CSS did not adopt TeX's glue. Although the issue was discussed, it was decided against in order to keep the VFM simple. Glue is very useful when breaking paragraphs into lines, but CSS leaves this problem to implementations. CSS allows, but does not demand, inter-paragraph line-breaking optimizations. Each box in CSS is, however, potentially richer than the boxes found in TeX since it can contain a padding, border and margin bands. CSS also borrows other features from TeX, including the em and ex units.

https://www.wiumlie.no/2006/phd/#ch-css

Håkon also goes into great detail on the historical context and technical requirements surrounding the development of CSS.

I think the reason the CSS box model differs from the TeX box model is because of the different problems they attempt to solve and various contextual difference the technologies developed under. Crucially, it's not because the authors were unaware of the TeX box model or because they thought they could do something better