Most exchanges are trading futures without having full access to coins. For instance have you asked yourself why Bitcoin Cash was available for trading on the first day, but it took exchanges close to 2 weeks to enable deposits? They are also using widely available software wallets and need to test the code first to avoid security issues. Such wallets for BCC were available fairly quick but the situation is different with BTG, that's why deposits are still disabled even though some exchanges allowed trading even before the fork.