As you say, the main reason is most likely low demand.
This causes poor economics in several ways.
For tax reasons they can not just tap of 5-10 liters at random and ship out.
This would have to be accounted for in some way and would probably cost more in manual labor and paper work than most are interested in paying.
Setting aside one batch for people to buy onsite would be a bit cumbersome too.
It can't be stored in a barrel, and they would need a seperate steel tank for the purpose.
Still keeping track of what's going in and out.
Usually excise tax is calculated by the bottling plant, and most liquid sold at the distilleries have been back and forth to bottling and in such way been taxed.
The exception is "bottle your own" directly from a cask.
And in those cases you register your filling in a log book that will be added up when the cask is empty.
This is a service they give on more exclusive tours, like at Laphroaig you get samples from 3 casks and then make a choise from which you want to fill a bottle.
But it costs, and I doubt that many would choose a 25cl bottle of new make for the £60+ they pay.
So to summarise, getting around legal issues is too costly to sell new make in bulk.
New distilleries often sell new make in an atempt to generate income before they can bottle legal aged whisky, usually with the excuse that you'll be able to judge the base of the coming product.
Otherwise there is the "evolution packs" of miniatures presented both from Kilchoman and Auchentoshan, with new make, 1YO and 2YO in miniatures.
The only well established distillery I know of that sell new make in other ways than miniatures, is Highland Park.
They bottle and sell new make, but it's watered down to 50% abv and cost £21 for a 35cl bottle.
That's £42 for 0,7 liter which is £7 more than what they charge for a 0,7 of the 12YO.
Why more expencive? Probably because it's made in smaller numbers due to expected lower demand.
So, clearly a novelty for those with special interest.
If you're interested in ageing, why not do as I did and take it all the way, ageing your own new make?
All the ingrediants you need are available at home brewing stores, and making a basic pot still is not that difficult and quite fun too.
1-5 liter casks are in my opinion novelties, they have way too small volume to surface ratio.
20-30 liters are minimum if you want any maturation and not just flavour influence.
I mashed around 100kg of malt and ended up with 2 casks of 20 liters 68% new make.
Cost wise, casks, still, barley, the lot... I think I ended up at around £15/liter (the casks at £150 each are reusable, and the still is certainly too, bringing the cost conciderably down if I want to have another go... problably as low as £5/liter.)
And it's starting to be enjoyable.
First one have been sitting 1 year next Sunday and it's been interesting to follow the evolvement.
Think I'll go draw a sample right now to toast the New Year