It's a strain on teachers when parents ask for advance work packets to accommodate their vacations. It takes time to put those together, figuring out what to assign the child before the class's lesson plans for those future dates have even been written. It takes often an hour or more per child, for a teacher who already has a heavy workload. And when multiple parents request this, as often happens nowadays, it just gets unreasonable. Especially when you consider the fact that many children end up returning to school without that work completed anyway.
I'm a teacher and our district now has a no-advance assignments/no make-up assignments policy for family vacations, because the numbers of them have gotten so high. The kids don't get zeros for missed work or anything, their grades are simply averaged without those assignments. In theory, their parents are supposed to catch them up on missed content, but in reality, that typically isn't going to happen and is still extra work for the teachers. With state testing in math, reading & science and strict accountability standards, we're held responsible for each child knowing the content, regardless of how many vacation days the child took while instruction was going on.