...the same reason children arent given serious roles in every other narrative medium?
theyre stupid? only other kids can identify with them? theyre not what producers think we all want to be like? they cant grow badass beards? theyre dumb? they cant have a love-interest of any importance without being really creepy? child actors are hard to come by? theyre morons?
ICO has an ambiguously young hero and is often considered quite the work of art, but what your asking for is a rarity in any medium, not just video games; you cant really be disappointed that developers dont take advantage of a unique of very difficult to pull off concept enough.