A native Japanese speaker said to me "見たことが ないから ～です。" to mean "because I didn't watch it, …." (we were talking about a movie).
As I have learned the sentence structure using から is "[reason: informal sentence] から [result: sentence]", I felt this sentence was too complicated. So I asked her if it was possible to say "見なかったから ～です。", as 見なかった is the informal version of "did not see". However, she refused and said that would sound strange.
So my questions are: why is the informal past negative form unnatural here? Are there other informal sentences that cannot be used in front of から? What is the pattern? Do the same rules also apply to other compound sentences (like と おもう) and other situations?