gismu != primitives
This represents one of the classic Lojban fallacies, and even James Cooke Brown believed it for a time.
There is nothing inherently "primitive" about most of the gismu, nor are they intended to be so. As an obvious example, mamta means female parent, and could have been represented as ninmu je rirni. But it isn't, because it is a sufficiently high-frequency concept that assigning a gismu for it made sense even in the Elder Days.