Cos mine can't and it's driving me mad! He used to have quite bad separation anxiety when he'd cry as soon as I left the room. He grew out of that a few months ago but he's still not keen on being left alone.
He rarely plays by himself. If I need to make lunch or do something in the kitchen, he'll come with me and hang off my legs. I absolutely cannot sit on the sofa while he plays in the living room, I have to sit on the floor next to him or ideally, he has to sit on my lap.
Is it completely unreasonable to expect him to play with something or do some drawing by himself while I read the paper for 5 mins?! Not sure if I'm just expecting too much of him...