Well it's really both. Definitely a great example of survivorship bias (we only remember the good books), but also it's so easy to publish a book these days compared to 30 years ago -- publishing is cheaper and "print-on-demand" means that tail-end publishers want quantity over quality. And publishers can do mass reachouts to potential tech authors all over the world (and they do -- if you have even a small blog with a "how to" tutorial on, you've likely been contacted by PacktPub, etc.).