[rant / ELI5] why do so many condo listings not include the square footage?

Title

I'm looking for a starter condo in San Francisco, and the number of listings in my Zillow feed lacking the square footage of the property is infuriating. Size is probably the fourth most important real estate factor (after location, location, and location). Why do so many listings leave it off?

Taking a quick scroll, somewhere between 25–33% of listings in my saved search have no square footage listed. I'm not going to bother eyeballing property size from the comically fisheye photos in the listing, especially at a time when there seems to be so many condos coming onto the market!