Once the workers organize as a union, the company is forced to treat everything as a negotiation with the union instead of the employees. The union views their job as to always ask for more and push back on the employer, so the employer is incentivized to hold back raises, benefits, and concessions until it's time for negotiations. Giving away all of your bargaining chips before negotiations start is not a good strategy.

When you see headlines about unions negotiating certain benefits or raises from employers, it's often framed as a victory that wouldn't have happened without the union. The reality is more nuanced, as the union's existence forces the company to play hardball during negotiations so they have something to negotiate when it comes time to do the negotiations.