Why do I not get double value for words which I use gold tiles?

What makes you so sure that you do not? The score which shows in the top right corner already includes any bonuses gained from gold or silver tiles. 

Yes I know the score is in the top right corner, the score doesn't reflect the bonuses.

It is possible, I suppose, but it seems strange that you are the only one to have suggested that this is true. Is there any possibility that you could provide a series of screenshots showing the erroneous scoring. I'm far from sure that this is not simply a case of you overestimating the expected score or misreading the result.