Here's why:
* Interjections are words or phrases that express strong emotions or sudden feelings. They are often used independently from other words in a sentence.
* "Thank you" is used to express gratitude and is often used independently, for example, "Thank you for the gift."
* While "thank you" can be used as part of a longer sentence ("I want to thank you for your help"), the core function of "thank you" remains to express gratitude.
Let me know if you have any other grammar questions!