Se people don't know it's wrong. Some people say it because they hear other people say it. Some people say it because they think it sounds a little bit amusing.
I'm doing good. Many would consider the above incorrect because good isn't "supposed" to be an adverb, although nowadays most dictionaries accept it as one by reason of its wide use. In informal speech, it's OK to use good as an adverb, but in formal speech, well is the right choice. I myself usually never use the word as an adverb. I think it sounds awk