Well, GNU grep was last released 16 months ago, and the last change to its master branch was 4 weeks ago: http://git.savannah.gnu.org/cgit/grep.git
FreeBSD's grep was last updated back in August: https://github.com/freebsd/freebsd/tree/master/usr.bin/grep
OpenBSD's grep was last updated 11 months ago: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/grep/
Oddly, it looks like the Darwin grep was last updated in 2012: https://opensource.apple.com/source/text_cmds/text_cmds-99/g...
Strange that Apple would be shipping such an ancient grep.
Maybe they ceded this part of the OS to Homebrew? I know I never try to update anything stock in the OS. It's so much easier/faster to `brew install xxxxxx` than mess with the OS which might get overwritten with an official update anyway.