The editor with a map loaded is usually 2GB+. Couple this with building a map and you're over the 3.2GB limit of 32bit, easily. If we released a 32bit version it would be almost completely unusable without crashing from being out of memory. To build one of the official maps on Production Lighting Quality REQUIRES 12GB of memory.
As games get more advanced so do the tools and hardware required to build those games. This is just a simple fact of development. Most companies don't release mod tools because they don't expect the community to have the hardware to required to develop custom content with their tools.
Edit: Also, with 10+ machines on our Swarm, building maps on Production takes around an hour. All of those machines have 2.6ghz Core i7s. If a single machine were to build a map, it'd probably take 10+ hours.