They signed a fancy magical pact under the supervision of a WIZARD'S ADJUDICATING BUNYIP. Can't break that shit under the threat of bodily destruction of their MASTER's choosing.

For example, having all of their blood removed from their body. Or having all of their bones removed from their body. Or being turned into some small Creature. Or being turned into a massive slide puzzle that an adventurer may have to solve before they can become Normal again. Or having themselves erased from existence from the past, present, and future.

All worrying possibilities.