There's also no reason why you have to literally write everything yourself either. You can find open source licensed packages, read them to understand them, and then copy them into your config. Doing everything from scratch is a waste of time unless you enjoy the process (in which case go nuts).
It's roughly equivalent to trying to discover all of our scientific knowledge yourself from scratch vs taking "for granted" the knowledge discovered by your forebears. There is no shame or disadvantage in doing so.
Also, a critical objection:
> Writing your own packages is the best way to learn Elisp
Absolutely not. Reading a language is crucial. If all you do is write, you will pigeonhole into weird practices and generally fail to improve. Only by reading stuff written by others can you learn, as you're exposed to what other people do right and wrong, both of which will be different from you.
Of course, writing your own packages is also necessary, but not sufficient alone.