I believe Yudkowsky has put forth a theory that somewhere in our environment there is a "GLP Supercharger" that's causing the opposite effect of the GLP-1 agonists.
If a lizard bite can make you thinner, maybe there's a metaphorical lizard bite that's making us fatter.
Some people get fat just looking at food, and this may get worse with age. Slow metabolisms, bad genes can play a role. This does not necessarily explain rising rates of obesity over time, but it can explain how some people become obese so easily despite not eating that much or why obesity is so hard to treat or the high failure rates of dieting.
If a lizard bite can make you thinner, maybe there's a metaphorical lizard bite that's making us fatter.