Also remember that plastic model building is much less common than it was in the 1950-60s. When I was a kid, almost all the other boys I knew built them, and seemingly every store sold plastic model kits (even the local drug store) - now days you seldom see them in anything other than a dedicated hobby shop (and at least around here, those are few and far between
). Most of today's kids are far more interested in video games and the like than they are in building something with their hands. So despite the soaring population, the market for plastic model kits is much smaller - with young kids a small portion of that smaller market. Much of the interest now days is among adults - who are also looking for much better quality and detail than the 10-year-old who bought a simple to assemble kit 60 years ago.
More expensive to create and produce - with a smaller potential market means Airfix, etc. need to charge more for the end product to make a profit.