It's one of the differences between the Isekai and the Portal Fantasy. Sure sometimes the protagonist ends up staying, but the portal is in principle always there. Think Outlander where at this point they're popping back to the present for toilet breaks.

Heck sometimes in the more Isekai-like variants they start up a roaring cross-universe trade.

But ultimately that adds a sense of responsibility that Isekai lacks, because you never know when the FBI is gonna start wondering where all the gold and jewels are coming from your "technically not a smuggling operation because you technically don't cross a designated border."