Why do many Americans think they invented everything modern and of relevance?

Because those few are brainwashed and under the lie of a belief that this IS the best country in the world; thus, everyone else must live in huts or some other form of poverty

Lol. Some people don't realize that many "modern" inventions are really just updates on ideas that have been around for centuries. An example would be da Vinci sketching plans for a ornithopter flying machine in the 1400's. Well this design was eventually revamped into a helicopter, which if you look up who invented it or when was the helicopter invented, you'll get several answers placing it around the early 1900's.

I've actually heard, and see on almost every label of everything I buy, that China is the real place where things originally get invented. Maybe America wants to take credit for it, I've actually heard when China makes stuff, America pays them for it and brings the stuff here to America. So basically, China is the real inventors.

Because we generally don't think about where things came from or who invented them. We're a buffet culture that thinks it's fine dining.

Arrogance.

Superiority/Pride?

America is full of dumb people, made dumb courtesy via government and society.