I love how people like you reason. "Nah man, the probability that mankind built that is really low. So I'm guessing it's aliens. Mankind just isn't smart enough to do that".
1) Occam's razor. You might find it unlikely that humans built the pyramids, so you involve an entity that is even less likely to have done it.
2) Seriously underestimating humans. "It's way too advanced, humans couldn't have made that!", they say while typing on their highly advanced computers, inspecting images on their illuminating screen. Images made with small contraptions that capture light by photographers in giant, metal flying machines. "But people back then didn't have all that!", they reply. It's true, they didn't. But that's because technology was less advanced. People back then weren't dumber, they just had less tools. In fact, I'd dare say people back then were way more inventive and resourceful than the average modern man. Not to mention in way better physical shape. A whole goddamn legion of hardened slaves tends to be a strong workforce, a whip on the back a good motivation.
Can I explain how they built the pyramids? Nope. Then again, can you explain how a tablet computer is made?