Worth mentioning there can be any number of reasons for someone not to bring a carry on.
On our last trip DH came down 3 days after DD, DGD and I. I brought everything he'd have in his carry on in my suitcase so it was already at the resort with me. He was able to travel with only a small backpack that held his book and other personal immediate needs for the flight. So when he arrived he hopped on Uber and joined us at the park, dropping his backpack in a luggage locker, saving a lot of time.
Some people throw caution to the wind. They don't agree that dragging half your stuff on the flight is necessary. Their stuff, their risk. Just because it's not how you'd like to travel doesn't mean it's not how they'd like to travel.