Because each additional story shared add contents to your friends feed ? And some things you don't want to share are the one that would interest your friends the most.

("friends" in the Facebook sense, of course).

And you don't have to click to share. You just have to do something in one apps linked somehow to your facebook account. Or leave a comment on a third party site. Or..

Yes, you can be attentive to all these, but that's not what I would call easy for everyone.