Because davening to Hashem is like talking to the one you love – you need to keep up the relationship.
And usually, Hashem does want to give us what we want, and He wants to be kind to us, and davening can make good things happen faster than they would otherwise.
But davening is not about what you get, it’s the connection to Hashem.
Imagine if the only conversation you had with your spouse was, “Could you please take out the garbage? Could you please make a lot of money? Could you please discipline the 6 year old?”
And you never just enjoyed being with your spouse and talking about your life. Isn’t that the wrong way to approach it?