It's probably more that it's not causing substantial deaths, more GP and hospital visits.
But also that it's hard to pin on an exact piece of advice - a car seat is a car seat is a car seat, but shade/protection can vary.
The best thing to do with a baby in the sun is wherever possible keep them inside altogether, which is more straightforward advice than exactly which shade/air circulation is ok.
(Lots of baby advice centres around 6m/1y guidelines precisely because it's easiest to get sleep-deprived parents to remember the information - it's not as if babies suddenly go through lots of exact and significant changes at 6m/1y precisely. Lots of it is actually weight-based, but they know that huge numbers of parents won't have the nous/the resources to know that precisely, so they give an age where 99% of babies will have crossed the weight threshold etc.)