Why do we need to believe to receive God’s promises?
A promise is void if the person who promises knows, that the one to whom the promise is addressed won’t believe.
For example if I were pretending to be a madman who assumes to be much more rich than I am and I promised you 10 billion dollars in my imitated rave, then my promise to you is void because you would assume that I am a madman and thus I would not expect you to believe me.
In the same way, if you don’t believe a God’s promise in Bible, God has no obligation to fulfill this promise for you.
So to whom Bible is addressed? It is addressed to these who believe. If you don’t believe, the promise is not addressed to you.