A lot of people don't care about cars but will still own one to drive to work every day. It's just a tool.

Its not. They just dont care if something is open source or not. Maybe they would happily use a free, closed source library instead.

Contributing back, or even announcing to the world that such packages are used is not even a thing.

When I said "entirely closed source projects" I meant that literally - some projects have no open source elements. They're written in closed source languages that you have to pay for, with closed source libraries and frameworks that you also have to pay for. They tend to be things like control systems in small industries, unusual embedded systems, and so on.

The software industry is big and hard to make general statements about. "All developers will use open source code" is not true.