If evolution is a gradual and continuous process, how can we say humans came about 200k years ago? What is so special about that point in time?
You're right that evolution is largely a gradual progression, but we don't have the luxury of having that gradual progression in the fossil record. We'll have a fossil from, for example, 350 KYA and another from 190 KYA. The older one is clearly not modern human and the more recent one is. So the age of modern humans is set at approximately 200 KYA, especially since genetic calculations confirm that age.

Now, if we find a 400 KYA fossil that is clearly modern human, then there'll be some 'splaining to do, but so far, nothing like that has happened.