Because he's japanese. In japan, there's an old fashioned idea, still, that men are supposed to be glorious and powerful while women are supposed to be meek and petted on and treated as a luxury - not the end prize.
Misty/Kasumi is just a flash in the pan. She and Ash will never get together, unless they go the route where he "comes home" to her after he's won ALL the pokemans and become the pokejesus. She'll be waiting for him with a cooking apron on and the camera will fade to black as she says, "Welcome home" and smile.
Japan's predictable as fuck.
