Unions here have a "strike budget" where if the union supports the strike, they pay a portion of your salary. Some can also give you loans.

Yes, they do. But for many low paid jobs, you need all of your salary to make the ends meet. And the unions don't have unlimited budgets either.

One of the major reasons why strikes typically last only a couple of days, is that the unions start losing their support when their members' personal financial loss exceeds the perceived importance of the issue.