In general we ask what you’re doing, even if we can clearly see what you’re doing, to show that we are caring and engaged.
By asking we are acknowledging that we see you there (to wave indoors is just stupid) because, you may not realize it, but there’s a time limit. If we walk into a room and you see us before we see you, there’s a good chance you’re going to think of something for us to do. “Whatcha doin?” not only sounds innocent, it also triggers the “he’s such an idiot” reflex. Science has proven that as long as you’re thinking that we are stupid, you can’t simultaneously be thinking that something needs to be scrubbed, picked up, lifted, cleaned, repaired, moved, retrieved, talked to or decided upon.
A simple, “hi”, while acknowledging that we saw you, will not trigger the “he’s such an idiot” reflex.
This adorable and devious ignorance was not actually our idea. It was the cat’s. We’ve watched for generations as women have cooed over the cute curiosity of the feline as it timidly sniffs everything or darts its eyes about the room after a bug. The whole time you’re thinking, “that’s soooo cute” and not “that thing just shit in a box and I’ve got to clean it up.” The cat – evil and brilliant.
As for your specifics:
a) am clearly changing a diaper – I’m not sure if I could visually identify a diaper so I’m going to skip this.
b) am obviously using the toilet – We’re trying to get you to say, “poopin.”
c) I’m making retching noises with my head buried in a garbage can? – Seriously, you’ve got your head buried in a garbage can? Whatcha doin’?
I hope this helps. And if all I did with this answer was make you hate the cat, well, I’m okay with that.