Probably because it's a big hassle. But of course there are projects that work that way or similar. Some big open source projects like Magento have an open source core but sell an enterprise version with closed plugins.

Unreal has its source code freely available to use but when you sell enough stuff you have to pay.

And I have seen several developers offering their open source software for sale to companies that don't want to contribute back or don't want to open up their integration.