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"?
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
I am very sorry.
You are very welcome.