Handy,
The difficulty is in the tolerances. Te geometry is fairly simple, but the tolerances that are required are quite tight.
The relationship between the trigger parts and the doodad (whatchmacallit on S&W, thingee on SIG) on the striker is pretty tight. If you start adding up the tolerances in the frame, the sear, the rails, etc on the lower half, and the tolerances in the striker holes, the rails in the slide, etc. They can very quickly add up to .020" or .030". Looking at my G17, I suspect that the rails are ground in the frame. I have a G21 in the safe at work, I will throw an indicator on it tommorrow.
That Glocks run as consistently as they do is a testament to both the design, and the manufacture. Glock does an extremely good job.
In a hammer gun, the tolerances between the slide and the frame never get added up, because they don't matter, as far as the actual firing goes.
As I said above, its not the basic geometry thats the problem, its the tolerances that must be applied to that geometry.