The idea that the subjects of Greek myth were originally real people and real events was put forward by the 4th Century BCE Greek philosopher, Euhemerus and the idea is referred to as euhemerism. Although folklorists have long discarded the idea as not fitting the evidence and not the way folk traditions have been observed to manifest internationally, the concept frequently appears. It is the subject of questions on this sub roughly once a week. Simply put, there is no evidence of this occurring with ancient myths.

Indeed, there is every evidence that this could not be the case. The Greek pantheon and many of its stories find echoes internationally from India to Ireland. This cannot be explained through after-the-fact literary diffusion from Greece, Instead, it seems likely that it is the result of a diffusion of tradition, likely tied to Indo-European languages, and occurring long before the Greeks emerged as a literate people. The origin of these stories is lost to prehistory, but it is inconceivable that all the ancient people who told these stories over the expanse of Eurasia were remembering real people and their actions several thousands of years before.

At the same time, we need to see aside the alternative that you propose, namely that it is "all just fictional stories passed on over generations." Internationally, oral narratives typically fall into two categories - fictional narratives (what we call folktales) and stories generally told to be believe (what we call legends). Greek "myths" should more appropriately be referred to as legends: they were stories told to be believed. That does not mean that by the time of the golden age of Greece that there were not skeptics. People asking questions about these sorts of things have always been ubiquitous, but the Greek myths are legends that over the generations were repeated to be believed.