I am pretty sure it isn't that easy. Financial feasibility is the number one constraint on any economic enterprise and am pretty sure if it was possible people would do it. I remember some time ago that a hobby shop owner told me they sold about 1000 Subaru and Skyline shells a year (in Adelaide, SA) from Frewer at 30AUD a piece compared to a handful of others. Frewer makes them in New Zealand and the shells are pretty basic with simple vague lines, but very durable so they're ideal for bashing.
As for durability I think the problem is with the lexan composition which is probably quite variable from supplier to supplier. In principle lexan should be tough as hell (just try to bash a Coca Cola bottle and you'll see) so I don't know why some shells are durable and others aren't. Chemistry of plastics on the other hand is a closely guarded secret and one difficult to unravel for would-be copy cats because of the intricate lab equipment you need to do it. DuPont invests hugely in research and I suppose they don't like to lose money by making things easy to reverse-engineer. One example in point is Tamiya which makes a lot of stuff in China, but not the plastics which are still made in Japan (we're talking R/C kits) and my guess is that they are not sure they can QC properly something made overseas (plastic quality/chemistry from suppliers being the first suspect). Otherwise, as you say, injecting plastic looks as easy as anything but I suspect it's the stages before the injection (plastic material chemistry basically) that are more important rather than molds, teperature, pressure, etc.
Team Greasy Weasel
The best upgrade to any car is some driver skill.