Because that is how AML policies work. You're guilty until proven innocent, not the other way around. Exchanges/financial institutions are obliged to report suspicious transactions. Local authorities will determine if they want to do a follow-up investigation or not.
Why would a deposit to an exchange constitute a "suspicious transaction"?
As I tried to explain earlier making a deposit, followed by a withdraw, can be seen as a mixing
attempt, which is a money laundering
indication. That doesn't mean that OP is involved in illegal activities, trying to launder money etc. but it
COULD be the case. The fact that it
COULD be the case is enough reason, according to AML/CFT policies, for exchanges/financial services to ask for KYC/additional information or even report the transaction to local authorities. I'm not trying to justify or defend these policies, I'm merely trying to explain how they work. The exchanges are simply obliged to follow these policies, otherwise they're not allowed to operate.