What do you mean "dismissed"? They released videos of both those tours; how is that dismissing them? "& still are" - do you actually know anything about this band?????
This was a time when live albums were still relatively speaking a rarity. Plus it's clear to any dedicated Genesis fan, they were a band who'd only release a live album if they felt it had something new to offer and at a milestone in their career. The Mama and IT tours would obviously not in their minds have met that requirement.
They clearly felt they didn't need a "huge seller to bridge a 5 year gap". Releasing an album, even a live one, isn't some casual thing. It's a major operation involving a whole load of commercial, technical and promo machinery. At that time they'd have had no time or appetite for it.