I was not talking about my current situation, mind you. I made a career as a software developer, which is one of the few professions that allow for flexible schedules and working before completing your formal education.
Europe is quite diverse, so your experience might be very different, but here part time jobs are really scarce, and as I said even if you find one it is quite likely that you won't be able to fit it with your uni schedule.
Most degrees have a schedule where theory is taught in the morning and practical classes in the evening or the other way around: For chemistry students for example that would be theoretical classes and laboratories, but an ill-conceived attempt at uniformity made it so that even for fully theoretical degrees like math you still have those "practical laboratories" where you'd basically just be going through exercises. Attendance is mandatory for everything.
> It might be hard to finance all your university expenses by yourself, but you can definitely cover a lot without any real impact on your studies. Just be more effective with your free time
While you are right that you can ease the load (summer break jobs are certainly doable), even if you're able to do that, for many people just taking care of part of the cost (or even all) isn't enough. It's not only about higher education having a cost, it's also about the cost of opportunity, since their potential income is needed at home. A year studying might be a year that the family can't afford turning the heating on in winter, or mom being unable to go to the dentist, or your sister not having new clothes. The student sees that every single day while knowing that if they just forget about uni and get a job wherever, they can start solving those problems straight away. It's clearly not the best long term decision financially, but the humane factor is crucial and hard to explain unless you've been there.