Should I not have won a culture victory by now?

Yes, you should have won a culture victory. The requirement is that you have more foreign tourists (297) than any other civ has domestic tourists (251). In this case you would need 252 foreign tourists to win. It seems that quite a few people have had this bug since release, but perhaps not enough for Firaxis to be able to fix it.

Suggestions for what to do next:

• Try loading an old save from before you 'won' and see if it triggers correctly
• Report the bug on the Firaxis forums: https://forums.2k.com/showthread.php?4296383-Bug-reports
• Accept the moral victory and continue playing (or quit)