If engineering were lower status work than a lot of people think it is, it would provide a more viable explanation than the effect of a conspiracy.

Men in engineering on average aren't as smart as they think they are, and given the perceived barriers, mostly it is exceptionally smart women go into engineering.

The underlying problem is that engineering is lower status work than its participants believe it is. Women in it recognize this and switch to roles that provide the benefits of equivalent or greater status for an equivalent or lesser amount of effort.

Sexism does sound more interesting, but reality tends to be less dramatic.