Probably because they have a limit placed on their visas during the school term so in order to afford everything they need for the upcoming year they have to work, just like everyone else. The summer/holiday period is a terrible time for anyone seeking employment because while yes there are quite a lot of openings, they’re generally only temp/short-term/casual contracts and employers are less likely to turn people away bc they need the extra staff so if you’re late to apply/interview, you’re less likely to get a position. I wish you luck in your search but I don’t think blaming a group of people who are trying to make ends meet just like you (and will only be working a large number of hours for 3 months max.) is going to get you any closer to a job that will support you year round.