And they do it to trivialize consciousness. They can't explain subjectivity so they claim a light switch is conscious and ignore the hard problem. It is just another misdirection like giving a fancy latin or greek name to something they can't explain somehow makes people (including scientists) think it is a scientific explanation.
Probably. I suspect they also do it because -- like I said earlier -- in their eyes, panpsychism is preferable because it's bottom-up, just like materialism. Perish the thought that it might be top-down, that the universe might be the result of intention on the part of a conscious entity.
It probably also enables some of them to hang on to materialism, because they can make consciousness just another property of matter, like spin or charge. But you're right, it's all hand-waving and there's no obvious mechanism by which particles with tiny amounts of consciousness can get together and gradually evolve into organisms with comparatively enormous amounts of consciousness.
I suppose they could argue that the other properties of matter, such as charge, are what facilitate chemistry, and that with complexification in chemistry, complexification in consciousness could hitch a ride. James Tour, a renowned organic chemist, however, dispels the hand-waving, often very humorously. Just search for him on YouTube -- and ignore his Christian beliefs if you're not sympathetic to them. What he says about chemistry in particular and biology in general is still very valid.