From the few known examples, self-propelled ammunition accelerating outside the barrel means it's relatively slow when leaving the barrel, which both makes it less effective at short ranges and causes accuracy issues.

Caseless ammunition is maybe a closer thing, but also tricky (heat dissipation is harder, difficulties in making ammunition robust enough). H&K made a military variant once, but no military adopted it (Germany almost did, but with the end of the cold war it wasn't considered worth the cost)

Even adopting improved versions of existing systems is a big process for militaries, switching to something entirely new for something as fundamental as the standard rifles needs to have really big and obvious benefits to be considered.