If you bother to google "Finnish teacher's union" you'll find that 95% of the teachers are members of the same union. This is much the same as in the rest of the Scandinavian countries.

Having a union means that the union can set standards, it can discuss directly with "management" (e.g. the government), make sure that the teachers can get the resources they need, and it is important in handling any problems which might happen. The collective bargaining part is important, but only a tiny part of what a union does.

It is only in the US and 3rd world countries that you expect teachers to buy paper and pens for the pupils from his or her income.