A not-so-insignificant number of FOSS developers are well able to make quality UIs, but decide to charge for their more polished creations.
Between having to make a living somehow, and not reaping a whole lot of other personal benefits from open source audio development, it takes a very special kind of person to publish these contributions in the first place. Once they're published, generally with its UI defined in code by a developer person, they're not necessarily easy for a designer to edit.
Nor is there much of a steady community around most of the plugins. So many are "publish, feature-complete enough, move on" kind of projects.
As always, be the change you want to see in the world.
Between having to make a living somehow, and not reaping a whole lot of other personal benefits from open source audio development, it takes a very special kind of person to publish these contributions in the first place. Once they're published, generally with its UI defined in code by a developer person, they're not necessarily easy for a designer to edit.
Nor is there much of a steady community around most of the plugins. So many are "publish, feature-complete enough, move on" kind of projects.
As always, be the change you want to see in the world.