I presume you are speaking as a consumer rather than from a server-side perspective, since much larger capabilities/capacities have been available to the professional/business world for quite some time.
So, I'll give a consumer based view:
I feel the root of the situation is in several parts - The first is that, while 64 bit computing is clearly the fastest growing segment of the desktop, when you look at the segment as a whole it's still a small percentage of the overall.
The second is that it takes a few years for software to catch up. The huge majority of apps on the market today are still 32 bit, designed to run on 32 bot OS's, and therefore generally keep within 32bit limitations. About the most you may see is that newer games have Large_Address_Aware capabilities. Not a bad thing - Even if a game "only" goes from being able to use 2GB to 3GB, that's still a pretty massive 50% increase. What's a development cycle for a game? 2 years, or thereabouts.
But when you have 8GB installed, (I do), then the application is still not anywhere near approaching the limitations of your computer. You'd have to run multiple memory intensive apps in order to take advantage. Something like Gaming, encoding video (or 3) in the background, plus your mail, browser, ICQ/Chat, VoIP program, and playing music, plus a little F@H for the icing on the cake... all at once...
Power Users may do stuff like that, but the great majority of users stop somewhere areound game/mail/browser and leave it at that. So there's not a lot of demand for truly rude amounts of RAM on the desktop at the moment. Though it's clearly coming.
Rather, In My Humble Opinion, much of the the impetus to move to a 64 bit OS isn't sop much RAM related as it is GPU related - 512MB has become almost a 'standard' config for video memory. More serious gamers will push 1GB on a (260/280... 4870... x2 cards) single video card, and the truly rabid will use 2GB and more for CrossfireX and 3 way SLI.
That is an awful lot of memory mapped I/O, and on a 32 bit OS you'll reach addressing limitations very very quickly. Take the needed 500~750MB just to run the system add 1GB for your UberCardOfTheWeek and you're stuck at about 2GB of RAM at a time where the newer games you want to run on your awesome GPU can make use of more than 2 gig of memory. And if you want some RAM to do things like... you know.... run the rest of your computer, then you need an OS that can provide the necessary address space. That means 64 bits.
my $0,02, anyways