They are a little bit, because collective action is very hard. If everyone chooses to participate and pay the cost of organizing it is worth the cost and a union will happen. But if a number of people choose not to then everything falls apart, as it is no longer worth it for anyone to pay the cost and the union can't function. Often the benefits of a union are also a public good, meaning everyone gets the benefits of a union fighting for better working conditions whether they pay the cost or not. If so, then there's little incentive to pay and the union will have trouble keeping itself alive.

Unions need some external extra incentive, like the state forcing them to exist or subsidizing the cost of organizing, to exist long term.