Unions are basically legalized price fixing. What happens is that the union negotiate a "fair" price, and then all companies decide to pay no more than said "fair" price. See for example (original is in Swedish):
The problem is that the numbers that gets published by unions in Sweden are taken as law by employers. You don't really know what unions are like if you haven't heard your employer say "We can't give you a bigger raise due to our collective agreement". And since basically all other employers follow the same guidelines you can't get competing offers for significantly more. There is a reason why salaries are very flat in Sweden.
Another way to see it, collective bargaining goes both ways, ie both workers and employers will come to a joint agreement. So if we created a FAANG engineers union and created a joint pay-scale for them, then that would basically be equivalent to the non poaching agreement often derided in discussions like this.
Not all union models have sector bargaining and it certainly doesn't work for professional unions - and I am not saying that European unions really get the needs of m&P members and need to change.
I suspect your posting in bad faith here and in the usa SAG minimum rates doesn't effect the higher rates that successful actors get.