Even if you THINK you are indispensable, even if you are 100% CORRECT that you ARE indispensable, plenty of gamblers exist in management, and will happily try and route around you. Maybe it doesn't work out every time, but it works out often enough to encourage companies to keep doing it.

It's so much easier to fire one person than the entire department. It's really hard for your manager to replace you when your coworkers say "naw we aren't doing their work without being paid their salary". And guess what? If you're a cunt, unions can, you know, not strike! They can 100% stand by as someone gets fucked over. Ask any member of a teachers union.

Companies don't jettison entire profitable divisions of the company to kill Unions in the crib (walmart) because they DON'T get the workers a larger slice of the pie lol. Companies don't play you anti-union propaganda on your training day because unions mean they pay less for talented workers lol. If unions truly reduced the price of real talent, companies would start unions themselves!