Well, it was in civ 5 (and is essentially the same victory mechanics here), so maybe they thought it would be until they discovered they balanced it differently
But I think wonders generate a lot more tourism in Civ 6 (especially early wonders), and so basic wonder-spamming can go a long way towards a tourism victory. Compared to Civ 5, where having your wonders help with a tourism victory meant passing UN resolutions and building hotels, etc.). Additionally, the "double all tourism yields" was in a 'dead-end' tech (the internet) in Civ 5 that you had to specifically tech to. Now now it's in computers, so it's unavoidable on the way to a space race victory.
I think moving the 'double all tourism yields' from computers to telecommunications (or giving it it's own dead end tech after telecommunications, ala Civ 5) would go a significant way to cutting down on accidental tourism victories. Maybe also cutting wonder tourism output in half and adding a '100% wonder tourism' civics card near to end that people have to use.