Ex-teacher but of primary and I think the answer is the same. I can think of several off the top of my head.
1.) They think because you are a teacher (vocation!) you love it so much that you cannot think of anything you'd love more than to go on holiday with the children. Especially their darling child who is truly a delight and you should feel lucky to get extra time with them. In fact, as far as they're concerned you would be happy to pay to go with them.
These are the same people horrified about striking teachers/doctors/nurses etc because they should do the job for minimum wage and deal with the awful conditions because they should care about the kids/patients etc.
2.) They think you are going to have a lovely, relaxing holiday where you photograph orangutans, bathe in waterfalls, drink cocktails on the beach, with no comprehension of the stress and admin that comes with a residential or any consideration that it might not actually be your kind of holiday or that you would prefer to go somewhere with your own family etc.
3.) They think teachers work 9-3 and are on holiday half the year. No amount of pages of hundreds of threads will convince them otherwise. So they think it's only fair you spend your holiday working day and night and see it as a sort of payback for living a cushy teacher life (hah!). They would love to go to Borneo with their own work. Funnily enough when they go away with their work, they aren't responsible for keeping a load of teenagers alive 24/7 without a single minute break. And of course, other than a few meetings the time is theirs to relax and do as they please.
These are the same people who insist that the teacher retention crisis is caused by lazy teachers or bad teachers and tell people to leave if they don't like it. Then in September post threads asking why their child doesn't have a qualified teacher. After all it's an easy job. They tend to stop replying when it's pointed out that all the teachers who have left in droves and gone into other roles have not come crawling back to teaching.
4.) Manners have changed and thank you is apparently now only something you say if someone goes above and beyond and not for something they get paid for.
Presumably they don't thank hairdressers, checkout staff, waiters or taxi drivers either. These are the same people, I assume, who leave their dog shit hanging off trees in plastic bags and in my mind are beyond understanding and best avoided.