my question remains, I don't see a reason why it's not worth spending $15 or $20 to implement 2fa on this current forum
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....