and I don't completely buy the "they don't sell" argument
then you don't have a lot of knowledge/experience in disc sales.
standard weight discs (145-150g, 165-max) outsell tweeners by about 50:1.
according to rules of economics, they shouldn't even make tweeners due to the lost opportunity cost of not making a standard weight disc. they DO make them, but in much smaller quantities than that ratio, probably closer to 500:1.
i might have a real opinion on this, but i've found that about 95% of the people calling for tweener weights are people that are throwing the wrong discs for where they are on the learning curve.
if any of these people were asking for a 160g roc, gazelle, cheetah, eagle, teebird, cyclone, etc. i'd see a case for it. but no, they are asking for 160g orcs, beasts, starfires, wraiths, etc. and throwing less than 325'.
there are some target demographics for tweener discs, but they generally make up less than 2% of disc golfers. since production is driven by demand, there likely won't be more of these discs until that demographic pushes closer to 10%.
i've weighed a ton of stacks of champ/star tweener discs and most of them are about 5g heavier than marked.