Because there is an implied stamp of approval for publications.
These days nothing is keeping people from publishing elsewhere if they want to.
These days nothing is keeping people from publishing elsewhere if they want to.
There has been quite enough censorship and paywalling of research that my taxes fund.
Since a good chunk of researchers are funded in part by the government, I guess most journals would end up having to follow this "publish everything" requirement (money is fungible and some of every grant goes to administrative overhead, so you could argue anything any most universities touch is funded in part by the government).
Most publishers publish multiple journals, though, so I guess they could follow your rule as long as they were allowed to open up the, for example, "IEEE Journal of Perpetual Motion Machines And Straightforward Proofs That P=NP."