Totally guesswork here, but I'd say Chrome has a lot more telemetry, profiling and tracking built-in and its users tend to use a lot more plugins, including things like ad-blockers that scan over each webpage and can be beneficial (battery-wise) or not depending on content. Safari users are more of a barefoot type. A power user is more likely to not be running Safari. And a power user may, well, prefer to sacrifice battery power to get the power they seek.

Besides, there's some precedent set in 1998 by a certain OS that "favored" their embedded browser over the competition, so I doubt Apple would want to tickle that fancy.