Yes, this is basically because it's undeliverable without doing some absolutely unbelievably stupid stuff. It'd skewer any idea of a trade deal with India (as Truss demonstrated), it would collapse our higher education sector which is heavily subsidized by international students, and it would cause massive worker shortages in both the City of London and the NHS.

It's one of those "people want lower taxes" - "Ok, which public services do you want to cut?" things. They want immigration down to the 10,000s, but no one actually wants to prevent any individual migrant.