Cats can steal your seat for a number of reasons.
They may be attracted to the warmth and comfort of your seat. Cats also like elevated places for a better view. Another reason could be to get your attention or to mark their territory. Your seat may also have your scent, which cats find attractive.
To prevent this behavior, give your cat a comfy place of her own, such as a bed or blanket. Play and interact with your cat to divert it’s attention.