I've seen some cases where this comes up.. and can think of a specific reason not to distribute the toolset.
What's the physics system used in-game? If you're nodding, then yeah you know the issue. If you don't know what it is, well.. in some cases the models etc might have some physics stuff baked into them for the engine to utilize. Hitboxes etc, weight, friction coefficients, the like, I'm not a 3d modeller. But anyways, what that means is that when you work, you might have a plugin for whatever 3d software to export as your personal engine format.. that includes libraries compiled in from your physics engine. Those libraries (and many others, ever seen a game using Granny? Same thing) cause a rights issue.. because you can't redistribute them as they're the ones you have to pay a developer license to get to use. If they gave them out they'd be breaking the license and would get pants sued off them for using the physics engine in-game. By the way, this can apply even if you own the full sourcecode to your tools and want to open them up. If they use proprietary stuff that can't be redistributed, they'll either have to strip it out to make the tools not actually usable, or just not distribute them.
So it doesn't matter, some of the people at Volition could WANT to give us the tools, would love for a modding community to spring up, it shows people love your game and want to stick with it, and people who love a game will share it with others to make everyone see what you built and get out some of what you put in. Regardless of that, they likely aren't legally allowed to give us the tools, so they just don't. I would bet if you emailed them and got them to give a specific reason why they won't give the tools, it'd be that, licensing restrictions.
Sadly this isn't that uncommon either. Modding isn't a big thing, there's very few games that actually officially condone it, everyone else, just like here, has to work in the background and reverse-engineer stuff and work on what we have. I'd LOVE the tools. We won't get the tools. They probably also don't have ones that can be pulled apart, say a "Volition Texture Converter" which is independent of the model exporter, meaning we can't even get the parts they could give us because they're tied up with what they can't.
(Edit addendum) What MIGHT work, however, and I'd say ask if you can find the right people to ask, would be some info on their internal file formats. If we use that info on headers/contents/etc ourselves, well, they didn't give out anything worrying. Even if it's finding say a coder who did the file formats and getting him or her to shoot off a quick email saying "That file? Yeah, six bytes of header, a hash table for contents, a file tree as archived, and a CRC listing" or "The export tool handled that but I think it's a header, a filename, then vertex data followed by texture mapping data and a block of physics data". It's not a full tool, it's some propiretory data, but it's stuff that is of low importance, won't be the same as giving sourcecode, and while still requiring someone to write a tool for it would be a great boon in deciphering and making a tool for it. Just a thought.