[zubat]

The market is fluid enough - which isn't to say it's in good shape - that developers who are decent can usually find a high enough salary to stay quiet and cooperate.

Unions arise when workers have a reason not to stay quiet and they lack recourse. But in tech the response to not getting what you want tends to be to raise funds and start your own company, or at minimum freelance. This is imperfect and allows vulnerable people to slip through the cracks. But again, it's not enough to make for a turning point, at least yet.