While I don't know this for sure, I'd bet my last gil that two things are true: 1) S.E. knows perfectly well how to make not just good, or excellent tab targeting - they can make it crazy good - i.e. fabulous targeting, and 2) The current level of such 'automated' assist is left as it is, which is to say, somewhat unpredictable, for the purpose of interfering with bot targeting. If any sort of automated targeting worked well, it would make the bot world crazy easy. So... S.E. had a choice, leave tab targeting in the "somewhat clumsy" category, and thereby introduce a level of bot exploit control, or turn it loose and watch the bot fun. Not a fun choice, because if we, the readers, were the S.E. engineering team we'd be some hurtin' puppies after looking at how it works now and not being able put the better stuff out there when we know we can.

Metaluna