I agree, you can't expect any insurance policy to cover a pre-existing risk without disclosure and being prepared to stump up extra premium. Trying to claim for a risk that was known at the time of purchase of the policy and not declared can result in investigations by the insurers anti-fraud department and the policy holder potentially being prosecuted and/or blacklisted for all other insurers to see.
To be fully covered it's usually better to purchase your travel insurance as soon as you make your bookings for travel related products.