Sparring without restrictive rules and without gloves so that anything can be worked, against sudden additional attackers, practical weapons, in different environments, small spaces, etc. and nonstop, being forced to fight through the pain of groin shots for example, stopping to reset only after reaching a 'kill point'.
Sparring with sanda rules doesn't put you under such stress and give you the necessary strategy to deal with it. You may be very skilled at moving around in circles, feeling out your opponent, and exchanging blows for rounds and doing things that don't take illegal responses into account. You may be a champion at that and have knocked out many opponents in the ring, but there will be situations where you can't use it and won't know how to defend yourself if that's all you rely on and think you are prepared.