But if a small shark bites your limb, it can rip just that one limb off and the rest of you can be saved (unlikely but not impossible, I'm sure I've read of a surfer who had her arm bitten off by a shark and survived because she managed to get away from it).
If a big shark swallows you, you'll be immersed in extreme-pain-inducing acid, unable to breathe (and probably out of oxygen, because one usually doesn't have the chance to take a deep breath right before being swallowed by a giant monster), with nothing to stand on or cling to as the walls are smooth and slippery, and probably drifted around as the shark swims. Even if you managed to stab the walls, you still won't managed to get out of them in time.