The gods must be... blind?

[Inziladun]

But when [the Valar] were come into the Void, Ilúvatar said to them: 'Behold your Music!' And he showed to them a vision, giving to them sight where before was only hearing; and they saw a new World made visible before them, and it was globed amid the Void, and it was sustained therein, but was not of it.

But the other Ainur looked upon this habitation set within the vast spaces of the World, which the Elves called Arda, the Earth; and their hearts rejoiced in light, and their eyes beholding many colours were filled with gladness...

I see this as just the difference between seeing something in a dream, and physically experiencing it.
Before, the Ainur had no knowledge of, nor indeed any use for, sight. Ilúvatar put the vison into their minds, which gave them their first 'preview' of what the Music would be like when it was brought into being. When they actually saw the realm of Arda made physical it was even more amazing to them.

x/d with Mithalwen