Travel creates new unidentified clusters, the tourists also tend to leave the area rather quickly => very hard to find and trace who originally caused the cluster, and who were in contact.

Tourists also tend to have much different riskier behaviour compared to the average native inhabitant of a city (eg nightclub hopping many days in a row)

If you're travelling a short distance over a mostly open land border then sure. If you're flying somewhere then it's a different story.

There isn't a magical virus barrier at the border. But there's a jurisdictional barrier there. Contact tracing across the border is much more painful than doing it locally. Our civilization-level immune system is bureaucratic in nature, so you can say that national border is a barrier for its antibodies.