In a transparent salary environment, the management is incentivized to pay everyone based on their socially perceived value, to avoid the "OMG Susie gets paid 40% more than I do for the same job," even if Susie is actually 40% more valuable than you are.
Which means management is incentivized to probably pay mediocre employees a little more, and stand-out employees less than they could have otherwise earned.
That is adults. Many people I've met that are bad at their job don't know that they're bad at their job. And the truly talented folks are going to be demotivated somewhat when they're being compensated for their 4x work the same as someone producing 0.8x.
Unionizing creative industries (I'm including most of software development here, as it's what I do) would serve to put a big safety blanket around the tons of mediocre talent there is out there, makes it harder to fire poor talent, and make all of our overhead higher (i.e. software costs more or profit margins are much smaller, both a bad thing for bootstrapping new products) without much overall boost to productivity or talent in the industry.
Mediocre/poor talent wins, everyone else loses. Doesn't sound particularly attractive from either side of the coin.
Which means management is incentivized to probably pay mediocre employees a little more, and stand-out employees less than they could have otherwise earned.