Those who come late may have to pay one in recognition that they benefit from the efforts of those who went before them. As a union member, you and the other members have rights under the union constitution to vote for the policies and practices of the union, including how much dues to pay. You get to elect your union leaders. The union is a lot more democratic than any corporation. If they were so concerned for your welfare, they could have raised your wages any time.
Individuals can beg while union members can bargain.
They pay dues to the Chamber of Commerce, trade associations, and other business groups that help advance and defend their interests, just as a union would do for you. They don't complain about those dues, because they know the value of being organized. Much depends on the policies of the union you are joining. Each union has its own rules about how dues are calculated. The rules are voted on by the members.