It's easy to finger-wag and I'm not telling people what to do, but maybe try to highlight why people are doing it:
Potrero is filled with moderate, multi-pitch sport climbing.
People simul-rap to save a little time (hopefully). 2-3 minutes doesn't sound like much, except you can get on a 23 pitch climb in the area, and well: then that time adds up. Gringos go down in the winter when the days are short (even in Mexico). Being benighted during a rap 1,000' up a climb looking for the anchors after a long day has its own issues.
Sport climbers tend to not use ATC-type belay devices all that much; rather preferring a Gri-Gri or similar. If you've ever belayed someone who's been hang dogging a pitch for an hour, you'll understand why. Can't rappel with a gri gri on a double strand rope, so what do you do?
Potrero moderate climbs are slabby and filled with vegetation. Getting a rope stuck in some sort of air plant lodged in a crack is almost a given. Knots in the end of the rope make this a bigger problem. Is being benighted with a stuck rope one of those situations you want to be in?
Given all these issues, simul rapping with gri gris down 1,000'+ routes without knots in the end of your rope becomes one style employed.
Are there alternatives to this approach? Yeah, there sure is. Using an ATC guide-type device works fine for these moderates, tying a knot in the end of the rope isn't the most annoying thing (until you/your partner forget to take out the knot - and remember it's 20+ pitches of raps, so it's not rare for someone to forget), and identifying the middle of your rope really seems a no-brainer.
It's sad to me that a lot of accidents in Potrero are from seemingly advanced climbers. But it just may be sloppy attention to details of the gear. Here's another accident that happened because the rope was not set up equally before the rappel + no knots:
https://www.climbing.com/news/two-climb ... co-mexico/