lord of the rings - Why are Frodo, Sam, Merry & Pippin going into lands "wholly strange to them" when leaving Bombadil?

There's really no reason for Hobbits to travel east of Bree. The next settlement in that direction is Rivendell; which no 'respectable' Hobbit would have reason to visit. They would have to cross relative wilderness, a dangerous road just to get to an elvish city that most of them would mistrust anyway.

There's also the nature of Shire legends. Hobbits seem to prefer local lore, such as stories about Bullroarer Took or family histories, to the affairs of the wider world.

So, we have fewer Hobbits going to Bree, the lack of incentive to go east (and risk being considered as odd as old Bilbo), and the general culture of the Shire being less likely to retain stories of the outside world. Together, I think these factors could pretty easily result in the lands even a short distance beyond Bree being practically unknown.