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 and why the Great Giza Pyramids (among others) were built, when they are not engineers or architects?
Because the engineering and architectural designs for the pyramids are no longer accessible and thus engineers and architects would be ill-equipped to explore that question (especially the "why" which implies judging knowledge about ancient cultures, beliefs, religions and practices, something that is ALSO beyond the purview of engineering).
The question of HOW they were built is an academic exercise for engineers, but not much more than that, because the engineer can only guess at possible techniques that would have been available or that would have worked the best. The archeologist has to figure out what the builders ACTUALLY DID, for which consultation with engineers will be useful, but not definitive in and of itself.
Instead they hold fast to the “tombs and tombs only” dogma that has never had a shred of convincing evidence to support it!
Actually, it's a fact that there's never been a shred of convincing evidence that the pyramids were built for anything else. They otherwise serve no purpose except to entomb the bodies found within them; if they served another purpose at some point, archeologists have not been able to determine it (and contrary to your claims, the academic world is inundated with theories as to what those alternate claims might have been, including some rather outlandish theories about Pharaoh's using them to fake their own deaths and/or pretend to be reincarnated in the personages of their offspring).
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.
I'm not sure who was "embarrassed" by that particular revelation, but I'm equally unsure that the geologists' findings are as relevant as you think they are, especially since geologic timescales are FAR too short to determine differing ages with that kind of precision. The only sources (not from ancient-aliens websites) I've found conclude that the body was constructed using different stone as the face, which suggests two different stages of construction for each.
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.
And yet it was the research of archeologists that lead to the discovery that astronomy was even relevant, hence they eventually consulted them themselves.
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
Actually, the problem with the "fringe" or "borderland" areas of investigation such as Atlantis and ancient aliens is that they are typically embraced by people who lack expertise on ANY of those disciplines. I can tell you without equivocation that Atlantis is of great interest to archeologists, and "ancient aliens" is a subject of interest to both archeologists and anthropologists. But the overwhelming lack of empirical evidence for either theory means few people within those disciplines take them seriously and instead relegate them to an era of fantasy (sort of like a high science school teacher who wonders if that half-witted student napping in the third row might have a life-changing epiphany and grow up to become the first man to walk on Mars. It would be awesome if that happened, but it probably won't).
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...
You're conflating two different things here. Skeptical ARCHEOLOGISTS are not the source of most of those criticisms; THEY criticize the theory more on its merits and the ways in which it contradicts empirical data they already have available.
The criticisms you cite are most likely to be encountered by other people who ALSO lack any expertise in scientific disciplines. Which, as I pointed out before, is why you need to be careful not to confuse "evidence that would stand up in a court of law" with "evidence that would be persuasive to posters on a message board."
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?
Because if aliens visited Earth in the pre-historical past, archeologists are best equipped to find out.
If aliens visited Earth during the historical past, archeologists and historians in combination are best equipped to find out.
If aliens are visiting us NOW, then astronomers and/or astronauts are best equipped to find out.
If aliens are going to on their way to visit us in the near future, then astronomers ALONE are best equipped to find out.
If aliens are going to visit us in the distant future -- once their planet has evolved industrialization and space travel a million years from now -- that's again astronomers with some support from anthropologists.
The only people who AREN'T going to be useful in solving those questions are internet hobbyists who lack a solid background in any of those fields or have experience in a field that is totally unrelated to them. Otheriwse, it's like hiring a dentist to operate on a ruptured spleen: it's just not the kind of thing that's going to yield good results.