are and have are two verbs in the latter sentence.they cant be used together.and "have "is equal to 有。maybe you can say:if there are more questions about it...it's about grammar,I guess everyone hates grammar,haha

"If you have any more questions about it..." < Correct. Yes. "if you are have any more questions about it... < Nope. Not correct. ( I added an "s" here but it is not correct- you knew that and asked WHY. I know) But I think maybe in sentence #2 you might mean: "If you find yourself having any more questions about it..." < This is correct. If you have an idea. < correct If you are have an idea. < not correct - do you agree? If you are tired... < correct If you are have tired... < not correct - do you agree? are (next to) have = not correct Why is probably due to some very technical grammatical reason that comes out of a book - YUCK. So, if you just remember that "are" next to "have" isn't correct, you'll have it. Or...maybe some transitive-pariticiple-dangling-phrasal-prefixologist will explain some complex grammar rule to you. But it's not necessary to learn what's right. Always learn the positive. Always learn what is right / not what is wrong and you won't waste any time.

Because it isn't. Check the appropriate lesson of your book.