I've been thinking about it, and there are a few things I can immediately recognize. There is definitely this feeling
that adding a new post belongs at the top of the content, which is perhaps, influenced by the chome-vs-content emphasis you speak of.
Over the years, I have been trained to expect the main actions of a web interface to be in the content area, while navigation typically resides in the header
. Using the + Share button, for me, would initially require a conscious effort.
It's also influenced by the time-sensitive nature of most of my posts. I often check that what I am sharing has not recently been shared, before posting.
Lastly, the in-content field inherently feels more likely to maintain it's state (drafts). I, irrationally, expect the popup nature of the + Share button to be fragile