I guess nobody expects random people to do others job for free. However, that's the argument offered for open sourcing everything as if open sourcing was the holy grail to end all suffering in the fsckin world...

Because they needed a product to sell and Nemo has a bit different UI paradigm, which is not easily changed. To deliver commercial product you need control over such things.

Another fact of life is that it's impossible to deliver 100% open source product these days. Some closed parts are likely to be open sourced, but it may take time if it's to be done in a way that benefits all parties.