It's not hard to hire an engineer that can take a ruby on rails project and run with it. There's mountains of tutorials and books about it. You don't need the best of the best for this.
Some people have decided that that isn't good enough for their company though.
No language book can genuinely teach you the ecosystem for that language. The ecosystem is many times larger than could ever possibly fit into one book, but the ecosystem is what provides you with the libraries you actually reach for to build things day-to-day.
Expecting engineers to self-teach from a book is practically begging for NIH syndrome and a rewrite in two years.
Some people have decided that that isn't good enough for their company though.