The Mac was developed by salaried employees, so was Unix and C and all the Xerox PARC stuff. The internet and the web was developed by people getting paid for it. Knuth is a professor, he didn't have to do his work after hours for free.

My impression was that most successful open source projects (Linux, Mozilla etc) are driven by paid contributors.

I'm sure you can find some examples, but depending on your opinion on what the "best software" is, I doubt most of it is developed by people working for free.