Why are there four Gospels, rather than just one?

Because they are all included in the canon of Scripture. What people say are discrepancies are actually differences based upon the different points of view of each different author. They didn't sit down together and compare notes. They each wrote about events they each experienced and each person has their own way of saying or even interpreting those events. Let's be honest...if all of the gospels lined up perfectly, word for word, skeptics would still be crying foul because of them being suspiciously TOO close to each other.