I'm not sure what makes it so entertaining. I know I think it's too childish even for me, and I'm barely a teenager. But the basic premise, while when you take a step back stupid, illogical and utterly bewildering (for extrapolation on this point, watch the South Park episode ChinPokomon I think it's called), is fun, and it's probably one of the only games to implememnt turn-based-combat well.
EDIT: Also children find it fun to enslave innocent creatures and pit them against each other, like cockfights except with magical monsters.
EDIT: Also children find it fun to enslave innocent creatures and pit them against each other, like cockfights except with magical monsters.