Modern Plays Not Modern Enough?

In the movie cinema we're seeing the most modern characters, plots, settings, costumes and effects. Why then is it that modern theatrical pieces are either old plays modernised or new plays based in old times. The only times i have seen a completely modern performance is back in high school (the past 5 years) when we were told to make a five minute improvisation. Why then can't we turn the concepts of a modern five minute impro into a full length play (I'm at the moment writing one so i'm not a hypocrite). I'm involved in Roleystone theatre only because of the people there because roleystone does nothing but old tripe. The newest play roleystone has done is COSI in which i was involved. It was a great play but it was the set in the 70's. We are in the new millenium now and it contains more issues and concepts to base a play on than any period in history, so why can we not use them in plays today? Comments?