Well because the atmosphere is much to thick to be able to see that far. A better question tho is how come on a clear day the curvature of the earth can't be observed from the ground - particularly on a boat in the ocean.

Yes, over a long enough distance, the atmosphere does obscure things. However, light travels in straight lines. Let me explain: if the earth is flat, this means that an observer could theoretically see any part of the planet from any other, regardless of distance; however, according to your explanation, the gas and dust in the atmosphere scatter light, which is why the sky is blue (which opens up other discrepancies). But if I had a powerful enough laser (we're talking spots on the moon power here), I should be able to shine it in everest's direction from, let's say the west coast of BC, and cast a spot visible to observers closer to Everest (lasers like this do exist, btw). This is because light travels in straight lines (and don't even think about bringing up gravitational lensing. The light curvature created by Earth's gravity is minute, and unnoticeable). However I can't, and therefore the Earth must be round.