It forces the firm to hire more employees than they really want. The union requires an all or nothing commitment. Either hire all that we want you to or we strike and halt all production. Since 1955, those numbers have declined. Now, about 1 out of 7 nonfarm workers are union members.
Unions are popular in industrial sectors.
The number of union membership has declined because employment in the industrial sectors has declined. Service employment has increased and unions are not as popular in service industries. Union membership varies greatly between states. With the numbers falling, unions have developed survival tactics. They have merged with other unions.