For some reason the equality seems to hold for most applications with a complex user interface. Reasons may include that it is a cross discipline thing that requires skills/people such as interaction designers, artists, technical writers, linguists etc that are less common than developers in the OSS community. It could also be that good interface design benefits from a centralized decision process which can be a bad fit for OSS that is usually too incremental and democratic for radical design changes. Normally there is no money for things like paid UI testing/focus groups.
That said, there is great free software with good UI/UX too - only they are more or less always backed by a large corporation.
A market with only free offerings will cause a certain category of actors to be absent. I would think that a market with fewer categories of actors would correspond into a drop in quality among some dimensions.
