They also get paid really well because Hollywood makes a lot of money and the national labour board ruled that most of the production companies in Hollywood HAVE to use that guild.
It's not clear to me who this technical writers union would be negotiating with, or why they'd have leverage over that entity. O'Reilly obviously has a limited pool of people it can find to write a book about SVG, but that also implies those people should have leverage to command more than a $4000 advance without needing to unionize.
The tech industry has a lot of money. Probably more than Hollywood, if you define "tech" broadly. Hypothetically, if all the technical book authors, how-to bloggers, and Stack Overflow contributors were to go on strike, they would have considerable leverage over various wealthy software firms.
They'd also have to somehow remove all the old how-to content from the internet and somehow find a way to stop scabs. Any up and coming developer anywhere in the world could find the basic specs and references needed to become a "strikebreaker" expert on a given topic.
The nature of our industry and the widely disseminated information makes me really skeptical that could ever happen, even if there was a strong will to do so amongst developers and technical writers.
I'm not saying it's that likely to happen, I'm just pointing out that all this "oh well, technical writing is just low-paid worthless work and always has to be" fatalism is misguided.
There are proven models for demanding higher pay for writing, but they require labor solidarity (and some measure of accountability from industry)
I suspect many books just wouldn't get written if advances had to be higher, there isn't a huge amount of money sloshing about in technical publishing.
It's not clear to me who this technical writers union would be negotiating with, or why they'd have leverage over that entity. O'Reilly obviously has a limited pool of people it can find to write a book about SVG, but that also implies those people should have leverage to command more than a $4000 advance without needing to unionize.