Unions operate financially like churches. They depend on the dues (tithes) paid by their membership. The more members you have, the more influence and power you have.
This incentivizes unions to never voluntarily reduce their numbers. The more mediocre the employees, the more protection they need from the union, and thus the more power the union has over the employee population.
The ideal union employee to a union boss, based on how unions are funded and run in the US, is a minimally competent drone that regularly pays dues.
Unions also consistently advocate credentialism over performance. If an employer wants to improve performance the union will fight any outcome based benchmarks, and will instead push for everyone to be educated and certified. In truth it would be best for the business to do both, educate and measure outcomes. What is more important, that every teacher in the US have a masters degree, or that every teacher be good at teaching. The union will always advocate for the former.
I like unions, particularly in the private sector. I wish they operated differently and were more pro-active in cultivating skill (like old school guilds) and less about body count.