The digital world pulls into itself as systems get easier and less user involved. A clap is the minimum amount of interaction anyone could give a story (short of no interaction at all).
BITD bloggers used all kinds of devices to get visitors to comment on their stories. You would up getting a lot of comments that were “great post” or “liked it”.
I think maybe people view the internet as a consumption device rather than an interaction device. Most things that aren’t social media have no way to interact that is readily accessible. For instance, needing to create an account with some other third party site just to give a comment or a signal of having been read.
Plus everywhere you look sites are begging for attention share, and there is only so much attention to be paid at any one time. Personally I always give more claps because I’m a writer and I sincerely appreciate the effort that goes into stories on Medium.