Earlier quoted context omitted.
I wish I could give you more upvotes. You are describing a somewhat hidden psychology, which I think provides a rational basis for much of "Not Invented Here" psychology. We tend to think that "Not Invented Here" psychology is irrational, but in fact, the loss of control over possibly crucial technology is an important cost, which makes all of us stop and re-consider whether we really want to use some software develo…
And it's not only that - Time spent not doing our own designs (and instead spent memorizing how to use magical frameworks) is time not spent advancing our technical understanding. It's a sad state when otherwise very intelligent people think it's bad practice to use plain C and a clean OS API that has been stable for decades (because that was somehow "magic" und impossible to understand and error-prone), and advise y…
That's a false dichotomy. The time it takes to "memorize a magical framework" is far less than the time it'd take to learn how to write code to do what the framework does. Consequently you can learn a framework and some other technical understanding in the same time it'd take you to only learn enough to implement your own version of the framework. In most circumstances that's actually more beneficial to do that. You'll be further forward in your understanding of the technical stuff.
You're also assuming that frameworks are written by individuals. They're not. In the case of some large frameworks it'd be practically impossible to implement what they cover on your own. You simply can't learn the underlying principles and then implement them all in code yourself.
It's definitely worthwhile learning the basics of the languages you use, and you should be working on things that improve your code and understanding as much as possible, but it's very likely in most cases that will mean building on top of someone else's existing code rather than implementing everything yourself.