Why would any sane person go through the obscene amount of effort of finding every old thread they can find to post in?

The 'middle of the conversation' you mentioned often has the conversants (from half a year, two years, or MORE) just gone. Any point they were trying to make is gone with them, and it makes more sense to start over.

'Hey Riley, remember six months ago when we were arguing about the price of rice in China? Well I finally thought up a retort, let's start going at it again!'