Masculine pronouns were very much the default outside of a few cases (like the "what are they doing?" another posted mentioned). You can say it's due to sexism which is a reasonable argument, but that doesn't change the fact that defaulting to masculine pronouns was the recommended (and most widely used in practice) style.

Seriously, read anything more than about fifty years old. The 1918 Strunk & White¹ stated, "Use the masculine pronoun unless the antecedent is or must be feminine." They argued against inclusive pronouns in the 70s when E. B. White (yeah, the mouse guy) was in charge of the style book, and didn't start recommending inclusive language until the 2000 edition.

The Chicago Manual of Style originally didn't mention it because it was so ingrained in the English language. It only started discouraging its use in 1982, and didn't condemn it until 2010.

Edit: I guess I should state right here that I'm not advocating for default male pronouns in writing. I think "they" is the best replacement. But I was taught "he" and adapted to "he/she" over the years. "They" really only seems to have become prevalent over the last decade or so.

¹That's Strunk and White the style book, not Strunk/White slash fiction.