I think it's just that it's iconic. If you had to pick a focal point [1] to meet in Cambridge, it would probably be Out of Town News for most people. But you're pretty much right. Probably no one's going to buy the newspaper from their hometown in Ireland there any longer.
The Curious George store actually isn't that old relatively speaking (from the 90s) I believe. I think it was in some way an offshoot of another Harvard Square bookstore (Wordsworth) which went out of business at the same time that lots of bookstores did. (There was a time when I would regularly make a weekend trip into Harvard Square in part to do the rounds of bookstores and record stores. Most are long gone.)
[1] https://en.wikipedia.org/wiki/Focal_point_(game_theory)