Em. Tourism has been a thing, complete with travel guides, tour operators, souvenirs made for tourists and tourist attractions for millennia.
Plenty of major destinations (holy places, resorts, etc) would bring in thousands of tourists at a time. The ancient Olympics, for instance, brought in tens of thousands of visitors.
Surely, those existed since ancient times, there's a reason why Romans had inns and waystations. There was some expectation to host strangers traveling as an act to please the gods, etc. but for any mass events there would be lodging at inns, camps, and so on... Not an expectation that everyone hosts thousands of strangers when they pour over for such events.
The scale is massively different, they didn't have millions of people coming through a city during the summer, like Paris, Barcelona, or Lisbon gets. Just Lisbon gets some 5-6x its population as tourists per year, it's in a very different scale to some tens of thousands in ancient times, which wouldn't expect the same relative level of amenities as modern tourists do.
Plenty of major destinations (holy places, resorts, etc) would bring in thousands of tourists at a time. The ancient Olympics, for instance, brought in tens of thousands of visitors.