Since, the sentence has "never" it must be present perfect, but whereas it's simple past. Why is it so?
You are mistaken: never does not require present perfect.
In fact, present perfect is very awkward here unless your great-grandfather is still alive when you utter this sentence. The present perfect is a present tense, and cannot ordinarily be used with a deceased subject†, because the subject no longer exists to sustain a present-tense predication.
† There are exceptions: cases where the writer or author of a still-present work is deemed to be metaphorically present and speaking through the work, and cases where the subject is not the actual topic—the classic example is "Einstein has visited Princeton", which is ordinarily unacceptable but becomes grammatical in a larger context where Princeton is the actual topic "Many celebrities have visited Princeton: Carnegie has visited Princeton, Einstein has visited Princeton, Madonna has visited Princeton."