I can think of a few reason's it's "poo-pooed" by mainstream modelers, Smeag.. One, it's totally imaginary and non-historical, so there goes 75% of the "real " modelers... Military modelers like myself, are often ameature historians as well, and spend countless hours (and not a few dollars) on reference and research. We try to carefully re-create minatures of vehicles and equipment, weapons, people, and places that actually existed or exist, and tell a story to others that both entertains and educates. Sci-fi modeling, in and of itself, is nothing more than the collective works of a few people's imaginations, and many consist of engineering feats and materials that don't exist, and likely never will...
It's not about a sci-fi modeler's skills, it's definately about the subject matter, since outside the films and flights of fancy of artists, there's no data to back up the claims of the modeler. I dabble in sci-fi occasonally because it allows me to get into the electrical part of diorama-building and allows me complete freedom to conjour up objects to flesh 'em out from a plethora of sources and material.. But I'm a military modeler first and foremost. This kinda puts me in a "Nomad" category, I'm equally comfortable with doing a Humvee in Iraq 2007 as I am with a Fokker Dr1 in 1917. Many modelers pick one military category and never venture outside their comfort zone, too... The jet guys look down on the Wire-wingers, who in turn don't understand the sling-wings, and the tread-heads consider everything with wings as just something to shoot at...
Still more model subjects that they actually have hands-on experience with as well... I model a LOT of artillery, since the 10 years of my 30-year military career was being a Redleg, and many of my builds reflect that. I doubt there are many, whatta ya call'em, gunpla crewmen?, out there, and I've yet to meet a former Colonial Marine or Imperial AT-AT driver...
I'm pretty sure there's a "geek-factor" involved as well... It's one tning to model some stuff from well-known sources of entertainment like Star Wars or Star Trek, but when you get into "Japanimation", you're reaching to a pretty small audience... With military/auto/ship modeling, eveyone knows someone who was in the military, has been on a boat, or drives a car, they know automatically, what the subject is and what it does... Hell, I don't even know what your talking about with "maecha", I assume it's it's some kinda Transformers/GoBots/Mech Warrior stuff, and that's a pretty small audience, compared to the rest of the modeling world... Cartoons, video games, and Super Heroes, they just don't make a popular modeling subject simply because there's no reality to compare it with, and it is, after all is said and done, a market that is geared toward the "kiddies", parents are the primary purchasers, but the kids are the targets and, as yourself know, are largely designed to played with...
However, if such things come to pass in your lifetime into the real world, you can say, "Hell, I was building models of those thing when they where just ink-drawings in a Tokyo movie studio."..