The problem is simple enough - it's about economy and practicality for manufacturers. With a fully tunable system like that, they'd have to individually set up each carb to find the right flow for a given engine. With fixed jets, once they find the 'right' ones for a particular engine combination, they can just get any monkey to assemble the carb using the jet/bleed/choke sizes they specify. And it's easier to keep in tune.

The SUs don't technically count, because the real a/f curve is set by the needle and the spring, the moving 'jet' is only to get the initial setting right so the needle profile works across it's range. They essentially only have to alter one setting to get them all to line up (so to speak) once the right needle/spring is found.

Of interest the Reece fish carb, and indeed the 'predator' (was hailed as the replacement for holleys etc when first made, but never really gained massive popularity) carbs are all more or less infinitely adjustable. Which is good if you have the time and patience and skill, but most potential buyers were put off by it because it's just too much to have to do to get to run well. Things like Holleys might be 'fisher price' in design, but generally if the carb size is well chosen, tben the out of the box metering blocks and air/fuel bleeds are close enough that with jet/powervalve/acc pump circuit optimisation, it's usually satisfactory for most users. To get really technical the hardcore tiinkerers will fully optimise even the holley, if it means fitting new high speed air bleeds, modifying the emulsion holes in the metering block, you name it.

SO the very short version - yes they do in fact exist,and their main drawback s that they are so inifitely tunable that some potential buyers would get lost trying to dial them in at all.;