Sounds like a chainsaw carb. Now to answer your question, I don't know but I can guess;

1) Fixed jets never change their flow and so are less likely to need resetting ever

2) I would imagine when manufacturing cars for a particular location all the cars would get the same carb jets then a quick run on the rollers to make sure the mix is within spec. Having to tune every single one would be much more expensive.

There are probably more reasons however there's no reason why you couldn't make your own adjustable jets and fit them to a carb. If your design can reliably lock the jet position you might have a nice after market product.

Just my thoughts