I've seen a handful of threads pop up today asking about why X or Y have not been implemented here on the forum, and in the interest of trying to save a long-winded explanation I'll try to keep it simple and concise.
Take a look at another user being responded to by Vod for asking about a similar implementation
It's probably just that little bit of money, plus a LOT OF PROGRAMMING TIME.
Theymos has modded this forum considerably. That's why it's such an old version of SMF - he can't upgrade it anymore.
A "simple" thing like 2FA could break a dozen mods he has made....
So the answer is security, efficiency, resource management, etc. There are only so many things that can be done before moving to the new forum software (which have been being worked on for years). It probably would take a long time or a lot of effort to program into the forum, then once it is properly implemented it might come with security flaws/holes, which lead to further problems or even simple compatibility issues with what has already been programmed into the forum.
It is a lot of effort and risk for probably minimal reward; there is not normally a time where copy + paste can be used and still make sense in response to the thread/conversation. So most of them will be thread-starters, and a lot of people around here already search little parts of your posts to see if it can be found elsewhere. There are already people watching, this gets called out fairly regularly and punished
severely.
