Yeah, I've been making this argument for years too.

Speedometer 3 is in the works, and it's moving in this direction. On multiple occasions we've done performance analysis of the latest draft version, noticed a particular chunk of code is hot, considered what it would take to optimize it, and then changed it because we don't think it reflects real-world usage.

EDIT: And I guess the answer for "why not fix SunSpider specifically?" is that once you've done the work to turn it into something worth optimizing, it no longer bears any resemblance to the original. It probably looks a lot more like Speedometer, honestly.