The real problem with "Official Support", as I see it, is that it would add to their overhead costs/work which would take time away from doing other things.

Say Larian wanted to sell the Cow Simulator mod for money (hypothetical scenario). Where might people go to buy it? There would have to be space made somewhere on either their site or another site which they link to. They would have to make sure the payment system is set up correctly. When things go wrong in e Cow Simulator mod, they have to take time to help with Technical Assistance. All of these issues are relevant even if it's a third party mod they're endorsing.

And then there's the dark side/thorn of the Gaming business, Pirating. With the way the file structure is currently set up, it would be ridiculously simple for someone to take an endorsed mod and distribute it as a free version. Going after these would take legal costs/time that could again be used elsewhere. All of these costs would detract from any profit they could get from another person's work.