Well, because when you want to get stuff done you really don't want to invent the wheel again. Why not reuse the work someone else has done before?
This is not to say that understanding web at a low level isn't useful, it certainly is. However, in my opinion it is better to for example build a simple mvc framework as learning experience and use something battle tested in production.
Reuse is good. But my fear is that people learn the intricacies of Rails over SOLID Ruby. The basics are worth practicing over and over again -- the benefits accumulate much like compound interest. Whereas there's no guarantee of Rails being around in ten years.
There's a certain sort of intellectual deference that is given to these frameworks (much of it has to do with how readily people accept things said by famous/rich people). For example, is MVC really the best way to write traditional webapps?
Why don't we train developers for this instead of constantly telling them they need to learn flash-in-the-pan-framework.js?