I would never say, "I am sorry very much" or "You're welcome very much", why is it that it fits well with "Thank You"?
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Thank you very much could be regarded as a short form of "I thank you very much", which is entirely sensible.
On the other hand, you might say "I am very sorry".
A possible reasoning is this: sorry and welcome here are adjectives, while thank you is an exclamation. You can add the adverb very in front of sorry and welcome to say