Why do some people not reply to "thank you"?

I've often wondered why some individuals seem to neglect the simple gesture of acknowledging a "thank you". Could it be that they simply don't see the value in expressing gratitude, or perhaps they're so caught up in their own thoughts that they fail to recognize the intention behind the words? Is it possible that they view "thank you" as a mere formality, devoid of any genuine emotion or meaning? Alternatively, could it stem from a sense of discomfort or awkwardness in responding, or perhaps a lack of social skills? Whatever the reason, it seems to be a widespread phenomenon that begs for a deeper understanding.