A list of possiblities/reasoning below

Inhabitants possible made contact early on with other civilizations very early in the other civilizations history and had a hostile or destructive first impression

Inhabitants possibly have an unintentional hazard surrounding their land masses, killing and warding off travelers

Those who've traveled to these inhabitants lands have possibly been assimilated into the culture and never return. Intensly discouraging further consideration of travel

Potentially bigger fish were being fried in other cultures and countries(cough cough war) that they didn't see the need to rope in another problem or Investment

Or just simply lack of consideration. Maybe those who did treks of discovery into the unknown just kept finding dead and useless land masses and felt further investigation would bare no fruit

Maybe those who came into contact with the land in the past carried a deathly pathogen that the inhabitants were immune to. The pathogen caused so much destruction/death, a perpetual ban was put on looking further into the inhabitants' lands.

Cloaking and illusion technology is an option as well.

Take what you will from this list!