There are modifiers, like policies that give you "+50% tourism from civs you have a trade route with"m which is kinda how it worked on civ 5. So the power of your tourism value to attract visitors is different in each civ you met, i really wish the game was more transparent on this point. In Civ 5, i knew what i had to do to stop (or slow down) someone`s culture victory: close borders, stop trade routes, denounce, declare war, pick a different ideology, invest in tourism myself, etc. But in civ 6 i really don't know what im doing, and culture victory (or defeat) always seem to come out of nonwhere.