Obviously we do. Don't copy paste 10 pages of source code unaltered and sell it as your own.

But that's something entirely different from small code snippets, changed and adapted to solve the same problem a thousand other people already had. Nothing else are developers doing going on GitHub, StackOverflow or any other website to find answers to their questions. That's not naivety, that's how coding works (partially). If you would have to re-invent the wheel everytime you build something new, good luck.