Yep, it's a browser vendor's reaction to a developer-tilted monoculture. On repeat.
On the plus side, User agent strings are moving closer to being useless as conditional branches for features. Hopefully, vendor-prefixes won't be far behind.
I guess developers just don't grok abstractions when it comes to the Web. They know their preferred browser, but somehow fall short of abstracting their development to "work on the Web" rather than "Works best in Chrome on an iPhone 5S."
Ever notice everyone identifies as Mozilla first?
http://webaim.org/blog/user-agent-string-history/