Like why is it “necro”?

Necroposting is posting on an old forum topic, which is not allowed. However, there are exceptions, like the TIMAC (Things I'm Making and Creating) and TIRAP (Things I'm Reading and Playing) forums if you want to revive an old thread. I'm assuming the “necro” part is to do with necromancers, someone who can revive the dead, or the prefix necro which means death.

Necro means dead in Greek.
Most english root words/prefixes/suffixes come from Greek or Latin.
More information:

Necroposting is when you comment on a post from a long time ago without adding anything to the topic. These can often be seen in the form of bump posts on a QaS topic from many years ago.

Note that necroposting does not exist on certain forums, if I recall correctly, those are Suggestions, Collaborations, TIRaP, and TIMaC.