There are estimated to be 4.4 million software developers employed in the U.S.[1] and about 150 million total jobs[2], so about 3% of employed persons in the U.S. are software developers.
About 11% of U.S. workers are union members[3], so there are more than 3 times as many union members as professional developers in the U.S.
Mostly public sector (around 1/3 unionized). Private is around 6%[1]. And after the public sector it's transport, utilities, telecoms, construction, manufacturing. Basically a laundry list of underperformance in the US -- important sectors (govt especially) choked to death by rent-seeking and complacency.
In software, sure, but to say there barely are any union jobs in the US currently is wildly inaccurate.