There is nothing in the culture from the great pyramid building age that suggests they built great pyramids. Obviously you can make a case that "Khufu" or "Snefuru" et al existed and then point out that the pyramids were named after them. But NOWHERE do they say they built them or displayt any knowledge of how they were built. The closest thing that exists to support them building them is "Merrer's Diary" which says they ferried stone from the Turah Quarry to Giza. The closest thing to suggesting any Egyptian knew how they were built is the presense of a tomb of the "Overseer of the Side of the Pyramid". There are a couple of apparent archetechs but further than this and things start depending heavily on interpretation. None of these facts constitutes proof that they built the pyramid. In light of the extensive evidence and logic that they didn't build the pyramids then it would certainly seem reasonable to suggest that perhaps they did not. The men who were believed to be the builders only two years ago have been ripped from the builders village. There's nothing set in stone and nothing that you can point to that says the Great pyramid was built in 2470 BC by Egyptians like Egyptologists believe. It's all ephemeral and as amorphous as a cloud. There is no science, no metaphysics, no record, no drawings, no plans, no bodies, no evidence of grave goods, nothing to support current beliefs. Current beliefs are a crazy quilt of guesses and assumptions that once seemed to make perfect sense but have not been supported by the facts over the decades.
How can you possibly look at the absense of any substantial cultural evidence for building pyramids, anomalous C14 results, and the rest of the physical evidence and suggest the pyramids were built as, when, and how Egyptologists believe? Nobody seems to like the facts but what disturbs me is that many of these facts are knowable but the testing hasn't been done.
What I believe is irrelevant. The fact of the matter is there is almost no evidence to tie the Great Pyramid to the time and culture that are believed to have made them. This is a fact whether you believe in little green men or the power of superstition. If they weren't made in 2470 BC then maybe it's because they already existed and were adopted.
You asked for reasons to believe the pyramids weren't built in 2470 BC and I merely provided some powerful reasons to believe they were not. I was quite sure you weren't going to agree so I didn't go on and on. Of course there are more reasons to suggest great pyramids or G1 are older but they aren't as good and are largely interpretation. G1 seems to stand alone in many ways so even if the other great pyramids were built by Egyptians then it wouldn't necessarily apply here.