I think some of it is that "union" has a different meaning in the US vs (eg) the EU. US unions are explicitly adversarial and tend to use the strategy (usually associated with capitalism!) of optimizing for short-term union benefit above all else, using brinkmanship and value-capturing tactics. EU unions, according to my weak understanding, are significantly more collaborative and more likely to be amenable to compromise if it contributes to the health of the corporations or institutions.

My naive view: in the US, unions are all about creating another set of assholes to counterbalance the existing assholes. In the EU, there's at least some thought towards "hey, maybe we shouldn't all be assholes?" (Or at least, not all of the time.)

That doesn't address your question of why it doesn't happen in the tech sector, but perhaps my anecdotal opinion is widespread enough to be added to your (already good) set of hypotheses?