Another factor might be that it is very difficult to judge your height above the water, especially with no land or ships nearby as a reference. The only thing you could use to assist with this judgement would be the size of the waves - but of course wave size varies so much that it would be virtually impossible to flare at the right height.
(Incidentally, sea-plane pilots have this problem, even when they are near to land, on "glassy water", i.e. on a still day when there are no waves. The technique used here is to flare before crossing over the shore, while you still have a good visual reference for judging your height. You then set a moderate amount of power, hold the attitude constant, and wait - making absolutely no attempt to flare further before landing.)
FFF
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Edit to answer Clear-Prop's question. I'm certainly not an experienced sea-plane pilot, having got the rating last month and not used used it since, but as I suggested above, this probably isn't very comparable to normal sea-plane operations. I can't think of too many reasons why you'd ever want to land a sea-plane that far away from land or ships - and the brain will subconciously use all this information when judging the height above the water.