The production costs associated with producing these custom spots - voiceover, editing, etc. - would seriously eat into whatever money the network could gain from selling these spots. Plus, if the themes from the shows these spots are associated with are going to be used, the network must license the music as well.
Given the option of going through all this trouble for no apparent benefit, or running a network promo that could be run in many other spots throughout the day, would you lay out the money?
Randy says yes, GSN says no.