[selectionbias]

>Why is the implication that this is due primarily to sexism?

Well perhaps because of the persuasive evidence the article cites from peer-reviewed journals. For example, in the paragraph below from the article:

"In one experiment, reported by Corinne A. Moss-Racusin and her coauthors in PNAS, 127 US scientists were asked to hire an undergraduate lab assistant and decide on a salary based on fictional CVs of equally qualified men and women. The scientists were more likely to offer the position to men as well as more hours of mentorship, and gave a lower salary to women, about eighty-eight cents to the dollar."