Just snobbery, innit?
The TL;DR musings:
I think the line is normally drawn at food that we consider ‘easy’ to make at home. Because of the necessity of proving dough, etc, bread-based meals usually take considerably longer even than something like lasagna.
(Possibly extra layer of snobbery there because readymade pasta is fine, readymade pizzabases aren’t seen the same way - but possibly just because of the quality of those foods?)