FYI, I used to use this capability directly from the Southwest website on a weekly basis (for more than 10 years) but the option suddenly disappeared about three years ago. I've been in touch with Southwest phone agents (who act like they never heard of such a thing) who then forwarded to SW tech support. It has been a dead end every time to this day.

BUT....!!!!
I decided to try opening the email confirmation from Gmail on Firefox (rather than my usual Gmail on Chrome) and LO AND BEHOLD ... the 'add to calendar' function suddenly shows up. I went back to Chrome to double-check and it does, in fact, appear to be a Chrome-specific issue.

The bad news: Southwest has been aware of this issue for years. The fact that it hasn't been fixed demonstrates that the inconvenience is not deemed a priority.