Because it feels good to them. They can't reach that spot by themselves so it feels good when someone scratches it for them and so raising their butt up shows that they are enjoying it and that they want you to keep scratching them :)
Cats scratch mirrors to mark their territory, stretch their muscles, and maintain their claws.
Cats scratch windows to mark their territory, stretch their muscles, and sharpen their claws.
Cats scratch on mirrors to mark their territory, stretch their muscles, and maintain their claws. Mirrors may also provide a reflective surface that intrigues cats, leading them to scratch.
Cats may scratch on glass surfaces out of curiosity, to mark their territory, or to sharpen their claws.
Cats scratch on windows to mark their territory, stretch their muscles, and communicate their needs or desires.
Cats scratch as a natural behavior to mark their territory and keep their claws healthy. They don't scratch to intentionally hurt you, but it can be painful if they scratch you accidentally. Cats may not feel bad about scratching you, as it is instinctual for them.
dogs + cats = scratch scratch scratch
Cats scratch blankets to mark their territory, stretch their muscles, and keep their claws healthy and sharp.
Cats scratch glass surfaces to mark their territory, stretch their muscles, and sharpen their claws.
Cats scratch their face to mark their territory, stretch their muscles, and keep their claws healthy and sharp.
Cats scratch mirrors because they see their reflection and think it's another cat invading their territory. They scratch to mark their territory and show dominance.
all cats do