Why exactly are the proprietary OS clients not fully open-source?

I checked the FAQ and the policy seems to be “proprietary OS = (partially) proprietary client”

The document seems to allude to the fact that the team’s “time and energy” is best directed to the open-source OS platforms, and so fully opening up the proprietary platform clients would drain team resources. At least that’s how I interpreted it, feel free to correct me if it’s not the case.

My question then is why couldn’t the team simply provide a “source available” release of the source code for the entire clients including the GUIs, or even under a FOSS license, without allowing community engagement/interaction with those specific parts.

It just seems very silly and weird to completely cut off public viewing of the GUI sources out of the desire to not have to waste resources on community support of those sources. Since support for the builds themselves is already offered, I don’t see why having the source being available for the public to inspect, for transparency purposes and good faith, is a bad thing.