Because in an open source environment the quality of the product is not defined by its commercial success but rather by the fun to create and share it. Therefore it is more in line with its intrinsic quality.
It's just "quality", and while it may be difficult to say whether it correlates better with the fun during creation than with sellability, the important thing is that quality doesn't correlated with sellability very much. There are way too many other inputs that influence whether or not people will buy something - like marketing, economies of scale and access to distribution.
(Though I'm pretty sure that this hypothetical Open Source McDonald's would exist. It, and other fast food joints, solve a legitimate consumer problem - getting hot and actually tasty food quickly.)
What is this intrinsic quality, and why does it correlate better with the fun during creation than with the decision by people to buy it?