When I studied comp sci around 2000, women were actively discouraged from continuing their classes by professors. During an oral exam, a prof told a female friend of mine that women had no place in comp sci. As a result, not many women graduated, so two decades ago, there were just fewer women in the field in general.

I'm not sure what exactly qualifies as a "legacy game engine", but given the small number of women who worked in comp sci when games were made ten or twenty years ago, and particularly in male-dominated videogame studios, I would naturally not expect to see a lot of cis women with experience working on these engines (or on related tech stacks) today.

This seems like a bit of a special case, rather than a general representation of women in software engineering.