why aren't masks from focus stack EDITABLE?

Photoshop doesn’t have a feature specifically labeled “focus stacking,” it’s sort of an unlabeled benefit of using the command Edit > Auto-Blend Layers on a set of limited depth-of-field photos taken at different focus distances. So the question is, are you talking about a focus stack built using Photoshop, or built using separate focus-stacking software and then sent to Photoshop?

If your focus stack was built in Photoshop using Auto-Blend Layers, then a focus mask should have been created for each layer in the document. And each layer’s mask should be viewable and editable using normal masking techniques, as shown in the demo below. If yours looks different, that’s why others have asked for screen shots of what it looks like to you, so we can figure it out. But the demo below shows how it normally works to edit the layer masks created for a focus stack by Auto-Blend Layers.

You can edit the masks, but tonal adjustments (to match exposures) are baked into the pixels and for some reason they do follow the hard masks. That’s why you see a ghost of the mask after editing it.

Yes, I agree, the whole focus stack algorithm appears outdated and never worked very well. I stopped using it long ago. It’s time for a full overhaul, if they can find time between other things.

I have generally recommended Zerene Stacker or Helicon Focus; don’t know if they’re still in business.

All that said, there can be an inherent problem with macro focus stacking if you don’t shoot carefully, and that has nothing to do with Photoshop. If you just adjust focus with the lens, you will move the optical center of the lens forward and backward, and that changes the perspective (the angle of view). To eliminate this, the lens should ideally be fixed and static, and only the camera body moved to adjust focus. This is hard to do without a bellows and focusing rail system, however. A compromise is to move the camera and lens together.

Focus stack masks are no different to any other mask and therefore should be editable in the same way. Check the options bar for your brush blend mode.
Dave

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Channels, Options Bar, …) visible? 

What are the Brush Tools settings (Blend Mode, Flow etc.)?