The employer is incentivized to hold these back in any case, absent some countervailing pressure (e.g. retention against competing offers, if they exist, etc.). This is why the largest raises (in tech, at least) typically come when jumping companies.
The countervailing pressure is that non-unionized employees are more likely to leave.
Most unions in the US place a high value on seniority. The more seniority you have, the more benefits you get from a union. Good for people who have been there for 10 years, bad for new employees looking to get jobs. This makes unionized employees less incentivized to leave and more incentivized to stay and strike. Leaving (even if for higher pay) would sacrifice that union seniority that many members value.
It's a very different negotiating dynamic.
SpaceX Blue Origin Space Systems Loral JPL Boeing Embraer Airbus Cessna Northrup Grumman Lockheed Martin General Electric Raytheon Boom Rolls Royce, Honeywell Eurocopter Bombardier
not to mention the adjacent fields of Green tech (wind turbines) Drones/UAVs
There are a lot of options out there
That’s literally the point of unions. To turn the employees into a monolithic block for the purposes of negotiation.
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.