It is not uncommon for characters to be able to summon creatures and even demons. The demonic spells start at 10th level and by 20th level you are summoning in some quite powerful demons.
So given the average life span of a demon why have none of them spent a mere decade or so researching a Summon Humanoid spell? A bit of hand waving and mumbling and ^poof^ there is a player character appearing before you. With a bit of opportune scrying and you summon the PC while taking their bath so they don’t have their +50 demon slaying sword and mithril armour of imperviousness with them either.