I think NASA does

Google has one, but it’s still very small at the moment.

Technology unions are common in public sector roles.

Probably because the culture is different in a few key ways:

1. Government workers rarely even get a cost of living adjustment, without a union, even when they’re critical. Politicians often have the final say, and often don’t care about retaining key staff. (Or actively try to lose key staff…) This leads to a situation where the Union has strong public support, because the Union’s motives are aligned with allowing basic government services to continue during political wind changes.
2. A government doing Union busting gets immediately called out as Fascism. The government telling you you can’t get together to talk about how the government should change - is not a good look.