I think you've touched on one big source of this problem, which is that many of these icons have been trademarked and saddled with all manner of usage restrictions and licenses.
As an entrepreneur or designer, am I going to waste who knows how much time interpreting the usage guidelines from Share This, or devote valuable space on my site to giving the Open Share people an attribution?
No. I'm going to spend 5 minutes designing something that won't cause future legal threats or trouble at acquisition time.
The thing about open source is that the way it is licensed means that anyone who has acquired the license before it is sold (such as Google's Android) can redistribute it as long as it follows the license requirements. As such, ANYONE can still use the Google's version of the three dots or if anyone had acquired a license to the original before it was no longer offered (pre 2012), can also relicense the work. Can any of the other share symbols be freely used?
The Android share icon has been around since (at least?) 2006 and was used a lot on websites, particularly Wordpress-based sites.
It was initially open source but then sold Share This and trademarked. Most services use the icon shape without ST's green button background.