No, you don't live in a vacuum. Newton was indeed standing on the shoulders of giants. But to go from there to saying you don't deserve responsibility for what you've done is a grossly illogical step to take.

You could argue that Zappa did everything because of circumstance. You could make a good and utterly meaningless case because of that, that everything is responsible for everything. It's good because it's generic enough to be obvious; it's meaningless because if everything causes everything it's the same as nothing causing anything.

In Zappa's case particularly: I know of no other musician that learned music through modernist composers and then translated that into rock music, who started off playing drums in a small band, took control of that band, taught himself guitar and became a virtuoso, and produced 90 albums in about 30 years in every genre imaginable. Nobody even comes close in that regard, and nobody has ever done anything like it. So, who's responsible for that genius? The record stores, for selling albums that caught Zappa's eye? Zappa's parents, for letting him buy records? The band members who ducked out and let Zappa take control? Those aren't genius acts. The genius is the thing that Zappa provided that came from him and him alone.

Most historical genii I know of attribute their greatest work to something outside of them. Is there a good counter to this observation?

Shakespeare didn't. Joyce didn't. Beckett didn't. The author of my favorite novel, Daniel Handler, is quoted as saying something to the effect that the only sacred thing about his writings is that he took the time to revise them. Bach wrote for the church but I've never read something saying that he was merely writing for God. The attitude stated here, that genius comes from an outside force, is incredibly rare. I've never heard anybody who's made anything truly extraordinary crediting anybody but themselves. The act of creation requires ego above anything else.