Why is it always stated in its accusative form?? Like even on bank ATM, it just states that. What would be the reason??
This would be my reasoning, too (although, as a native speaker, I almost never think about why I say the things I say the way I say them ... ;-)).
There are similar expressions that must have evolved from complete sentences. For example, our greeting is
Guten Morgen! / Guten Tag! / Guten Abend!.
I think this accusative is used because it evolved from a complete sentence, too:
Ich wünsche dir/Ihnen einen guten Morgen!
Ich wünsche dir/Ihnen einen guten Tag!
It typically appears in adverbial expressions of a small, definite amount of time, which is probably why you always see it in the accusative, as this is the case into which these time expressions tend to get put. If 'a moment' were the subject of a sentence, it would be 'ein Augenblick', as you'd expect.