I prefer the simple theory; they were flying long-range over a remote part of the Pacific; their radio was wiped out by tropical static, and their navigation wasn't good enough to sight Howland Island. They flew about a large ocean looking for a tiny island until they ran out of fuel.
Navigating an aircraft was seriously difficult until after WW2. In 1937 ocean flying was done by dead reckoning, corrected occasionally by sextant, with both supported by moving an antenna to null out known radio stations and thus triangulate a rough position. Sextants are harder to use in a plane than a boat and primitive radio communications and direction finding equipment likely made it too difficult for Earheart and Noonan to get good bearings in poor radio conditions.
Attempting to fix position with a sextant on dry land taught me that manipulating them is skilled work. It also involves a stopwatch and an accurate time source. You have to take accurate readings, and then sort out the maths and almanacs. There are numerous sources of error. I doubt navigating an aircraft by sextant alone could get you better than 40 or 50km from a destination. Not a problem in good visibility, but awkward if you can't see the ground. I wasn't accurate on dry land in good weather and can imagine it being much worse in a plane or small boat, perhaps having very limited time to identify a star and get a reading in bad weather.
Early radio equipment was also challenging to operate; much skilled twiddling of dials and drifting off frequency. For many years larger aircraft carried professional radio operators. Partly because they were trained to make the best of the simple equipment available, but mostly to reduce the load on the flight crew, who were busy at the best of times.
Worst of all, navigational systems weren't resilient back then. Flying an aircraft over the sea near a tropical storm means your dead reckoning is likely to be wrong. Clouds and buffeting might make it impossible to get a star sight with the sextant. And static caused in the region by lightning could make radio reception difficult as well. With only a compass to rely on you could easily be a 100 miles or more off course and be unable to find a small isolated island.
After WW2 came radar, VHF radio, ergonomic equipment, beacons, crystal control, and a host of other radio aids. Even though it's much less likely to happen with modern aids pilots still get lost!
Dave