Well, normally frameworks give you for free a standard set of practices that make it easier to build a secure and maintainable application, so it's normal that web development is being moved to frameworks.

I find that most web applications or websites built from scratch, normally in PHP, to be written by amateur developers that have little knowledge of security, maintainability, best practices, database design, etc.

Right it's the concept of "standing on the shoulders of giants" While there is educational value in writing a web app from scratch, if it offers no core value to your product you are not working in the best interest of the project. A lot of developers go through a "not invented here" stage where they write everything, usually this is a mark of immaturity of the developers part. Understanding and leveraging frameworks helps the industry evolve. If we take the automotive industry for example (because everyone loves a car analogy), at one time there where all kinds of fuel systems and everyone built their own fuel system for engines. Eventually the market consolidated on best practices and companies like Bosch packaged up those best practices into systems like the Common Rail. Now when a car manufacturer designs a new car they generally build on the framework of the Common Rail for their fuel system. Further the entire power-plant is usually an off the shelf framework that they utilize in their design. This helps deliver a new product on a proven and tested framework for powering it.