“Don’t reinvent the wheel” is such a bad bad bad advice.
It’s by reinventing the wheel that we understand the intricacies of the wheel and it’s only then that we become experts.
All creative work begins with imitation. If you’re a photographer, you try to imitate / emulate photographs that capture your attention and make you go “wow!”.
This is not limited to artistic pursuits alone. Take the case of a software developer. He/she begins by writing simple programs that have already been solved many decades ago: Fibonacci sequence. Many jQuery plugins exist but writing your own is fun, interesting and rewarding. “Code is poetry” and poetry is art.
“Don’t reinvent the wheel” gets thrown around a lot when people are trying to compensate on time. This usually happens where a person is characteristically after profits and impatient. Or when the pursuit is less that of the art of creating wonderful things and more that of making money (or fame) out of it.
There were many frameworks for CSS layouts and HTML components before Bootstrap. But someone did decide to reinvent the wheel. And the landscape has changed ever since (for better or for worse). Why do people continue to create CSS frameworks even after Bootstrap? Because reinventing the wheel is what gets us to a better place than now.
The only way to learn is to imitate great things before us. The only way to mastery is to reinvent the wheel.