There must have been thousands of printers (and paper makers, and typesetters, letter casters, printing press repair people, page cutters, sorters, book binders, etc., and so on...), if there were 200 million copies of books being produced in the 16th century alone.
That seems rather crazy to me, given that typesetting by hand is pretty labour intensive.
Anyway, the main question is about woodblock printing and how it apparently didn't cause a book revolution. I'm interested to know why?
Well, you have touched upon the answer yourself in your question. The simple answer is that the printing press, specifically movable type, was an incredible technological leap in the process of creating multiple copies of a printed text with as little labour going into the process as possible.
With woodblock printing you carved the entire page into a single block of wood. This required an enormous amount of labour just to produce a single page, besides the fact that the wood block itself had a limited endurance compared to the cast lead types.
So while to a modern eye the printing press with movable type can seem labour intensive, it was still a revolution compared to the existing technologies of handwritten copying and woodblock printing. And with the relative quick specialisation of the printing profession (some specialised in typecasting, some in typesetting, some in pressing etc), the process of printing texts in hundreds, sometimes thousands, of copies became relatively easy and cheap (it was actually the paper itself which was the most expensive part of the printing process until the development of wood pulp paper in the 19th century).
I would imagine labour and time. It takes time and skill to make a woodblock, and you are constrained by the size of the lettering you can carve onto the material. the wood also degrades after a number of printings.
It doesn't take long to learn to set type, and the size is smaller, being cast in metal, so is consistent and very durable. The whole process can be turned into an assembly line.
It's also a bit subversive - a woodblock of a banned or heretic text is proof of guilt, and requires the complicity of the carver. A box of letters in alphabetical order proves nothing, and thus fuels the printing of controversial pamphlets and texts - the whole thing could be written, set, printed and distibuted by a single person, with a guarantee of anonymity. in an age where people were executed for these things in unpleasnat ways, this must have been very liberating.
Also, typos. Can't fix those too easily on a woodblock. Makes porn easy and cheap to produce too.