This leverage all has to do with the product, how close you are to it and how valuable it is. FAANG product is valuable, but the programmers are still just creating it and usually a small part of it at that. Pilots are the product when the product is "get plane to destination" they are the 1 or 2 people providing that product (ok service). Athletes are the product of sports. It's very valuable and there's only a dozen or so of them to deliver.
Now, using the leverage is the difficulty to unionize. Athletes are a tiny group, pretty easy to organize. Pilots are a small group, also pretty easy to unionize. The fact they have to be licensed means there is a record for all the people needed pull in. Software engineers seem to be 5x-20x larger population than commercial pilots (quick/rough searches). They have no certification or registry organization and have no common affiliations. It's incredibly difficult to organize this group. There's also no regulatory capture requiring developers to be US citizens so, if you did unionize and tried to negotiate too hard the industry would just move away from the US so there's just not a lot of leverage this profession has.
Pilots also spend years logging hours in small airplanes, then regional airlines, getting paid relative peanuts, working crazy hours, rarely home, etc. before they land a high-paying job at a major national airline. And any hint of health problems, depression, substance abuse, etc. they fail their medical and it's all gone.
These "unions" in high paying fields behave more like guilds or cartels than worker's unions - they generally restrict supply. Athletes and Hollywood unions are sort of special cases too, IMHO. I don't think it's reasonable to claim that top earners in those fields earn so much because of their unions - they benefit from natural supply restriction of outliers.
For unions to be as effective in tech as for say pilots or doctors, you'd have to agree on a way to restrict supply (H1B restrictions, more licensing and credentialling etc) to give the union leverage. You have to control the supply taps and rate limit entry to the field.
I think it's hard to say if this would net out better for workers than the current arrangements, which are already the best in the world on nearly every metric.
It also seems like there's a timing issue - if tech workers DID successfully unionise enough to withhold a meaningful fraction of labour, the gains might ultimately end up in the market cap of AI companies via substitution.
Structurally the way it works is that the AMA or ALPA or what have you lobby for regulations that just so happen to limit supply, and heavily push back whenever anyone proposes regulations that would loosen it, usually on safety / quality grounds.
There are also revolving doors between the regulator and the relevant professional bodies.
I'm pro union in cases where employers are a monopsony and workers have few options - it completely makes sense for coal miners in a coal town to form a union to even things out. I just don't think US tech right now meets the conditions for it to make sense, the market is too liquid for employers to capture all the upside.
US tech workers have real problems and complaints - PTO, maternity leave, health care to start - but these feel to me more like structural features of the US labour market? It makes more sense to me that these should be subject to national regulation rather than specific advantages for tech workers carved out by a union.
Now, using the leverage is the difficulty to unionize. Athletes are a tiny group, pretty easy to organize. Pilots are a small group, also pretty easy to unionize. The fact they have to be licensed means there is a record for all the people needed pull in. Software engineers seem to be 5x-20x larger population than commercial pilots (quick/rough searches). They have no certification or registry organization and have no common affiliations. It's incredibly difficult to organize this group. There's also no regulatory capture requiring developers to be US citizens so, if you did unionize and tried to negotiate too hard the industry would just move away from the US so there's just not a lot of leverage this profession has.