Because of this, double-click became business speak for going to the next level of detail, digging into, etc.

The idea behind this name for ads was: this company makes ads relevant and compelling, so users drill into them and find whatever you want to advertise.

For what it's worth, because of the affordance you mention, even though users didn't have to, they consistently double-clicked banner ads, and most things they wanted to activate, even after they learned they only had to single click the blue underlined things.