What reading I've done of the subject has convinced me that the pyramids of Giza are 100% human in origin, and that current prevailing theories about their age and construction are right.
We have lots of records about the Pyramids being built. Hell, we even have the ancient equivalent of worker timesheets. We know where the workers made their camps, what parts of the year they worked during, and how much beer they drank.
The Pyramids are impressive, but they also represent the very pinnacle of an art. There are LOTS of pyramids in Egypt, and not all of them worked. Several collapsed, or had to be altered mid-way, as ancient engineers perfected their craft. By the time the Giza pyramids were constructed, all of Egyptian society was trained as a massive but fine-tuned pyramid-building machine. When a society puts its collective mind to something, incredible things happen. Ours put a man on the moon, in less than 8 years. That's pretty darn amazing.
Now the Sphinx is a slightly different beast. There's certainly some evidence to suggest that the Sphinx was NOT built with the Pyramids, but it actually much, much older. The head appears to have been recarved; possibly several times. There are strange erosion patterns. And more than one text from the time of Khafre refers to the Sphinx as having been uncovered rather than constructed. I'm willing to entertain the idea that the Sphinx may stem from an even earlier civilization whom we have yet to uncover. But there's nothing unbelievable about the Pyramids. Just the ideas of one of the first recorded geniuses in human history and a whole lot of people working towards a common goal.