Maybe the cat is just really tired and the dog makes a comfy spot. Simple as that!
It could be that the cat and dog have a close bond and the cat feels safe enough to sleep on the dog. Or perhaps the cat was playing and got too exhausted and just collapsed there.
The reason might be that the environment was cozy and the dog didn't mind the cat using it as a bed. Maybe they were having a peaceful moment together and the cat decided to take a nap on the dog.
It could be for cuteness or to show an unexpected and funny situation.
I think it's popular because it triggers our emotions of fondness for cute animals and gives us a sense of calm. Also, the way it's depicted might be unique and draw people's attention.
The cartoon cat is sleeping because it's the natural thing for animals to do when they need to restore energy. Maybe it's been up all night protecting its territory or having adventures, and now it's taking a well-deserved nap.
Maybe it's just tired from a busy day of adventures.
The dog cartoon could be sleeping because it's the time for its nap. Or perhaps it's exhausted from all the adventures it had in the previous scenes.
Maybe it's just tired from a long day of playing and having fun.
The dog and cat might have just gone their separate ways and continued to sleep or do their own things.