Theymos doesn't have a good number of programmers for such type of implementation and that could be the reason, a few months ago PowerGlove did quite well for the forum and implemented a good number of features and fixes for the forum. The 2FA alone is the best feature we've on the forum, and that's because of hard work of PG.
If I'm not wrong, in order to implement such features Theymos has to get some good programmers just like PowerGlove who might have strong grip of coding of SMF 1.1.19. I believe for such type of feature someone who's good at modifying SMF can do the job pretty well, but still it's easy to say such things then to do those practically. However, to be honest, such a feature if implemented could be very helpful for some of us.
The challenge is adding anything new to the current forum system would create security issues and the forum already has three hack accidents, after that theymos spent his decent time and efforts to code the forum software for good security since then. So he has big reasons to hesitate adding anything new, any cool feature to the forum software if it is not too important.
If it can be done by userscripts, forum community can do it, and not all SMF patches will be approved by theymos even there are community developers code such patches.