Very few of them were even useful for anyone but myself
Most developers learn and grow by doing - which means learning by making mistakes, googling their error messages, and looking at examples of other people doing what they're trying to do - which is why you should always open source your code unless there's a specific reason not to. If you've ever made something that works, then your cube would be useful.
I've never felt dependent on public code repos for my own career before,
I hope you don't actually believe this. The entire Internet, and computing itself, is built on the foundation of open source. This is like saying "why do I gotta pay taxes" when you and everyone you've never met has relied on roads etc. And that's just the basic example - the real importance of, say, public education, is that, while you personally may not have used it, many many many other people have - and their education has pushed the quality of your collegues higher - which pushes you to be better, either as competition or cooperation. This is the actually accurate meaning of "the rising tide raises all ships."
Even if you've never used Linux, or any open source software at all, the rest of us have, and we're pushing your job and your career to new heights.