Legally speaking because it's in your contract. Ethically it's because when you agree to a full time position you are agreeing to make your time available, with reasonable allowances. Again, it's completely fine to make a different arrangement if that's clear up front.
Unless you are somehow working them in totally offset hours, or you have _literally_ zero meetings and work in a completely asynchronous environment, then you need to disclose.
But that's not true for an overwhelmingly large number of people. You aren't just being paid for output, you are also being paid for time.