The answer is simple because he wants and can. Although, humans who have cats know that the reason why our cat loves to lie on our slippers is actually somewhat more complex.
Cats love warm and comfortable places, that’s why you will always see them lying near a window just when the sun comes in with all its splendor, or on the sofa, in bed or on that plush blanket that you leave on your bed. Since they sleep most of the day, they need a comfortable place.
But, there is another even more important reason. Research by feline experts indicates that these friends are highly conditioned by the smells they perceive. For them, the smell of another cat determines sexual status and they use it to communicate, to interact socially and also… to identify their territory.
So we could interpret two things: either your cat falls asleep on your slippers to define you as his territory, or he tries to replace the smell that your body sweat leaves in them for his, after all, that house, it’s his!
However, the truth is that cats are attracted to pheromones and substances that mimic them. So as the fragrance of your loved human is highly concentrated on his feet, many you see them playing with your socks, with your shoes and your slippers. They are like an extension of yourself to him, therefore they are comforting to him.
So the next time you see your cat lying on your slippers, remember that he does it because he loves you and you are very important to him. Of course, you would only have to watch that their cute little guts do not hurt your shoes.