"generalized" means the most simple code you can come up with. This simple code is then used thousands times from very different programs, and it never changes.
I think implicit in the point, most code does not go on to be used thousands of time (let alone dozens or even more than once). Even if some code is used more than once, making it the same and shared has significant costs as well.
Never said it is easy to build simple reusable code. Code itself must be simple but process behind devising it may not be. If the code is not simple it is more prone to error.
