Here's why:
* "Thank you" is a fundamental expression of gratitude. It's likely that forms of expressing gratitude have existed in human languages for as long as humans have communicated.
* The phrase itself is a simple combination of common words. "Thank" and "you" have been used in English for a very long time.
While we can't say exactly when "thank you" was first written, it's safe to say it was long before the mid-20th century.