The reason is that a magnetic pinch operates preferentially on charged particles, where the greater the charge, the more the effect. This means that only positive ions are compressed. But positive ions cannot condense, because of the electrostatic repulsion. Only neutrally charged matter can condense. Hence all that we can expect in a z-pinched charge stream is plasma.
It should also be noted that it takes relativistic velocities to get much of an effect. Hence a relativistic jet can get pinched, and thus can stay organized for extended periods of time. But it is also subject to relativistic particle collisions, producing the UV radiation by which we detect such jets. Such collisions slow down the jet. And since the magnetic pinch effect is a function of velocity, the reduced velocity means less of a magnetic pinch. And that means that the jet is not so tightly compressed anymore. Then it encounters even more friction, which slows it down even more, which further relaxes the magnetic field. Then the jet falls apart in a turbulent Herbig-Haro object. And HH objects are not stellar nurseries. So relativistic jets definitely do not pinch down into so much stellar buckshot, as the EU model has it.
CC: z-pinches consolidate like charges, but that can't cause condensed matter.
