A union negotiates a contract with an employer. The employer freely enters that contract, and by signing that contract is giving the written intention of following through with that contract.

That contract is mutually beneficial. As employer, you are provided with a pool of selected professionals. As a worker, you are provided with fairer compensation and benefits.

Right to work comes in and says those contracts do not have to be honored by the employer and they can break that contract because the law says so. Anyone they employ now enjoys the benefits of that the union spent time and money on negotiating, but the worker does not have to pay dues to enjoy that privilege. Without the union, those privileges wouldn't exist.

Any "rational actor" can see that the rational decision is to enjoy the benefits a union brings to the table, without having to pay dues.

The counter to this is the argument of freedom of association, saying employers can hire who they want. This sidesteps the fact that employers made the free decision to entire such contracts and chose to stand by its stipulations.

Unions do not hold a monopoly on workers. Workers are free to entire and exit contracts with unions or negotiate on their lonesome. Employers have a variety of unions and individuals to pick their employees from.