Originally posted by LDHZenkaiBecause none of the Celestials have shown the power that Scathan displayed.
And what makes you think that? When it's stated that the dreaming celestial was the strongest of them?
Tiamut may be the strongest of the 2nd host, but he is not stated to be the strongest Celestial. Scathan is also from an alternate future, so even if Tiamaut were said to be the strongest Celestia, which to my knowledge isn't the case, that would not show he is stronger than Scathan, who is not around so there can be no comparison.