Actually, it is very possible for managed code to do 3d games, the problem is the back engines. With .Net, for a brief period, there was a Managed DirectX wrapper to DirectX 9 by Microsoft. This was before the abstraction that is now XNA.
Being given total access to DirectX api's, .Net games work a treat. The best example I know of is www.entombed.co.uk, which is written in VB.Net.
Unfortunately, on the Java side, it is seriously lacking - mainly for the reason that DirectX isn't available for Java, and games programmers know and understand the DirectX api - why learn yet another api when you will be returning to DirectX?