As for alternative theorists not being archeologists; let’s turn that argument on its head. What makes archeologists (or anyone else, for that matter) think that archeology is the field qualified to judge how
the Great Giza Pyramids (among others) were built, when they are not engineers
? Instead they hold fast to the “tombs and tombs only” dogma that has never had a shred of convincing evidence to support it!
This is precisely what justifies the trend of the last decade or so, in reinterpreting the age of other monuments such as the sphinx for example, if they had consulted geologists
in the first place, archeologists might have realized that at least the body of the beast was probably carved much earlier than they thought, and so saved themselves some embarrassment later on. The same goes for Archaeo-Astronomy, if archeologist had consulted astronomers about the alignment of these and other ancient monuments, they would have come to quite different conclusions about when and why they were built, and what level of sophistication our ancestors were capable of at those remote times.
The problem with many such “fringe” or “borderland” areas of investigation such as Atlantis and ancient aliens is precisely that they tend to “fall through the cracks” left by the separation disciplines; and no single discipline by itself
is qualified to evaluate all the pertinent evidence. Therefore, it’s not so much a question of individuals having multiple areas of expertise, but simply that the experts from various relevant fields need to work together and exchange ideas more often.
More on topic, most criticisms of the “aliens did it” theory mostly boil down to things like the vastness of space and the limiting factor of the speed of light and so forth, which are the purview of astrophysicists, so in order to refute these alternative possibilities,
sceptical archeologist must speculate outside their field of expertise, as is their habit apparently.
So more to the point, why should we regard archeology as the sole field to look to for relevant answers about aliens, ancient or otherwise?