On the other hand, C/C++ makes it attractive to reinvent the wheel, or vendor the dependency instead. Rather than a single well-tested implementation in the ecosystem for something like sha256, you end up with every application having its own slightly-different, mostly untested, and essentially unmaintained version.

Applications still need the functionality. The need doesn't magically disappear when installing dependencies is a pain. If a crate has a bug, the entire ecosystem can trivially get the fixed version. If the Stackoverflow snippet a C app is vendoring has a bug, that fix is never getting in the app.