Here's why:
* All languages have a common ancestor: Linguists believe that all languages can be traced back to a single proto-language spoken tens of thousands of years ago.
* Language change is constant: Languages evolve over time, changing their sounds, grammar, and vocabulary. This evolution often leads to branches and subgroups within language families.
* Borrowing and contact: Languages are constantly influenced by each other through contact, borrowing words and grammatical features. This can lead to similarities between languages that are not closely related.
Even languages that seem very different, like Basque and Japanese, have been shown to have some connections to other language families. While it's possible that there might be some very isolated languages that have extremely distant relationships to other languages, it's highly improbable that a language would be entirely unrelated to any other language.