Athletic unions are generally there because the top leagues are monolithic, there aren't other places to go with similar compensation. It's sort of a different playing field.
The Top Companies pay X, and that sets the ceiling for most people's wages. The difficulty of getting into Top Companies means that most other companies know they can pay somewhat less than that.
Although it's claimed there's a talent shortage and in theory that would cause engineers to be able to negotiate for large salaries, most businesses just don't think that deeply about it. You can't negotiate with someone who just doesn't care that the math works out in their favor. Many managers in my experience have anchor points on salaries that they just will not go beyond regardless of the quality of the candidate. The salary spread for junior to super-super-senior developer is only about 50% (100k-150k or 120k-200k), whereas the productivity difference may commonly be much greater.
Lastly, there is de facto price fixing. Human Resources Departments do "salary surveys" of what all the other companies are paying and then try to pay their employees around the same amount. Even though this isn't official "collusion", the effect is the same. And if 1 or 2 small companies buck the trend, it doesn't change the overall market.
Okay, so you have 2 options to make dollar money as an engineer: Google & Facebook.
Apple, Microsoft, and Amazon pay well, but 10-20% less for the same level. There's a handful of others that will pay about 20-40% less. The average startup is paying 40-70% less.
And, sure, you'll always run into some startup that's paying one or two engineers a million dollars.
Stephon Marbury made more money when he left the NBA to play in China. Doesn't mean the average player can or does.
Anyone making league average in the NBA could play in Europe or China for 40-70% less.
Those aren't the only options anymore, Netflix is for instance famous for compensating very well, and large startups can be competitive too. https://www.levels.fyi/
