The call of the wild. I had several indoor cats and lived in a high-set house in Brisbane where leaving the windows shut was not an option (too hot, needed the breeze). Carefully selected those windows that had too great a height to tempt the little dears but Claudius discovered that if he jumped from the window to the fuse box to the ground, it was not too high. Others perpetually got themselves trapped on the window sill, unable to turn around. And then there was Bob, who snuck out one night and woke me by jumping up to the window sill, hitting the side of the house, and sliding down, nails scratching all the way ... not a good sound at 3 am. I think it's like dogs and car windows. tehre are interesting smells out there that taste different from that angle ...
What was really frustrating was that if they went out the bathroom window there was a ramp into a large and complex cat run with lots of ledges, textures and layers that they could explore ...