Here's why:
* Linguistic relationships are complex: While we can trace the evolution of many languages, some connections are more tenuous than others. Even languages considered "isolates" might have very distant, unknown relationships.
* Ongoing research: Our understanding of language families is constantly evolving. New research and discoveries might reveal connections that weren't previously known.
Examples of languages sometimes considered "isolates":
* Basque: Spoken in the Pyrenees region of Spain and France, Basque is often considered a language isolate. However, recent research suggests it might be related to the extinct Aquitanian language.
* Burushaski: Spoken in northern Pakistan, Burushaski has been studied extensively, but no clear connection to other language families has been established.
* Ainu: Spoken by the indigenous people of Hokkaido, Japan, and Sakhalin Island, Ainu is considered an isolate, but its origins are still debated.
* Korean: Korean is often considered a language isolate, but there are theories about possible connections to other language families, including Altaic.
Important Note: The term "isolate" is somewhat misleading. It doesn't mean a language sprang into existence from nothing. All languages have evolved over time, and it's possible we haven't yet discovered the connections between some languages, especially those with ancient roots.