There probably isn’t any logical explanation. It’s one of many aspects in which Star Wars goes more into science fantasy than fiction, hence the awkward situation.

From a Certain Point of View ESB reveals that Ben Kenobi's line to Luke about not being able to interfere in the confrontation with Vader was just a latch-ditch attempt into scare him into staying. So yeah, there's no real explanation at the moment.

I wonder if Force Spirit Luke still helps himself to some blue milk, or maybe that other kind of milk (from TLJ) which shall remain nameless.

Nope. There’s no explanation.

^^ That was also blue milk, wasn’t it?

Because it takes a massive toll and concentration of consciousness to even appear in the first place. Death in Star Wars is effectively leaving the Living Force and being scattering among the Cosmic Force. A “Force ghost” is just a trained Force-user’s (usually a Jedi’s) physical concentration and manifestation from learning how to retain consciousness in the Cosmic Force. A voice to an individual is relatively easy. Concentrating your self and all(?) your senses so that you appear in the physical realm takes a lot more effort. Actual interaction takes a massive amount of effort and so far, in Canon and Legends, is only seen on Force nexuses. Parts of Ahch-To and the Wellspring of Life are examples of this.

The living: *struggling in a galaxy wide war*

Force ghosts:

^^^ It was green milk