Everything in our physical universe takes time to manifest. Our mind and Universe can be compared to a little child and his old grandfather respectively. When the child wants something and asks grandfather for it, granddad (the Universe) just smiles to little child (our mind), because he knows that one moment after little kid will forget what he wanted and will ask for something else. But when a he repeatedly continues to ask for something, grandfather will kiss crown of the child's head and will make his dream come true.
In physics it can be explained as inertness (inactivity, stagnancy, heaviness) of matter or substance. If your mind is able to create images very fast, physical or real image of your mind will take time to manifest.
That is why exercises are so helpful, because step by step you are training your mind and body and do exercises repeatedly. Everyday we approach closer to our dream (goal, objective, wish).
I would like to say few words about this powerful Wish Come True exercise, that is very easy to do, but remember to do it every day as many times as possible.