Insisting on ASCII files means some kind of markup commands buried in the text.
We want the markup to be "declarative," that is, to say "what you want" rather than "how to produce what you want." Section numbering, generation of a table of contents, much indexing work, cross referencing, creating of the Info "node" structure, etc., etc., is done automagically.
Our starting-point choice for a "declarative" markup notation was LaTeX, because that's what we know (and so do a lot of other people). You could make the case that GNU's Texinfo format would've been better, esp. given some of our other objectives. Or: you could argue that we should've chosen a completely different notation, because some people look at our literate files and think they are LaTeX, which they are not.
The most important parts of our LaTeX-like notation are introduced in section See notation-tut, and the whole mess is exhaustively described in section See Command_reference.