At my previous job we ported our entire engine (MMO) to 64bit, and ended up running a 64bit client in a few places as part of the development pipeline in order to preprocess data. But, we would never ship a 64bit client to end users.
First, the performance improvement is very minor, and most PC games end up being GPU limited in any case.
Second, very few end users have enough ram to take advantage of a 64bit executable, and any engine that is even vaguely designed for consoles (which max at 512mb remember) is not going to be optimized for large available memory.
Finally, having 2 executables in your distribution pipeline is a giant pain in the ass. You can't only ship a 64bit executable, because then 30% of your audience can't run it. So you have to ship both a 64bit and a 32bit executable, as well as some sort of wrapper that runs the correct one. If you're an online game that means an extra multi-megabyte file to patch every time any code changes, and additional complexity in making sure the right version gets to the right player. It's just not worth the effort for the minor benefits.