NuSpock and nuKirk don't really look like the originals but at least they try at some of the mannerisms. And basically they've got an excuse - its a reboot.
The other cases have an excuse too: It's not real, they're hiring actors to pretend to be these people, and they'd be pretty foolish if they made physical resemblance a higher priority than talent. Obviously the ideal is to find someone who's as close a physical match for the other guy as possible (or close enough to pass as family if you're casting a sibling) while also
being the best actor you can find and the one who has the best chemistry with the other guy. But the odds of getting both of those things at once are extremely low, and if you have to choose between resemblance and talent/chemistry, obviously resemblance is going to take a back seat. Hell, I take it back. That's not even an excuse, because there's nothing to excuse. It's just basic, common-sense good casting. Resemblance is nice, but it's not more important than getting the best performer.
Although, granted, sometimes there can be too little resemblance. I definitely would've preferred Hanks to Cromwell as Cochrane.