Just imagine a world where there was no voice for workers at all. Employers could just pay what they wanted and work you as long as and however they liked. Some people in this world would like this.
The beneficiaries of such a system would be the bosses and in some cases, company owners. The people already earning millions each year. They want to destroy unions so they can implement such a system in part or full so they can earn more and you can earn less. This is greed and these people are driven by it.
A good union leader will always want to oppose this greed. For me a lot of it is the fight against selfish people. If you think we should have a society where greed and selfishness flourish, get rid of unions.