Possibly a lucrative contract. Let's face it; yes, professional 'rasslin does have its hazards, where people can fall improperly while doing the more risky moves, other times your "opponent" might be incompetant (see the bios on Brock Lesnar and Bill Goldberg for this), and end up hurting you, but the chances of you getting hurt aren't as much as having to go toe to toe in the ring with someone who is honestly trying to send you to dreamland.
IIRC, Ken Shamrock and Dan Severn were both affiliated with the WWF for a fairly long period of time.