Because there's a difference between designing for mass production and designing to a specification and a price... I can design and build a one-off that'll be perfect...but it'll be a one-off... if I want to design it so it can be assembled and tested by a team of trained monkeys and is bullet proof to its environment it needs a lot more rigour. I cut my teeth on troubleshooting manufacturing problems at GEC-Marconi on head-up displays and the like... it was rare to get a batch that actually had 100% good ones, in fact there were on occasions batches that were close to 100% failures... With logic circuits its obviously easier to get it right (though the number of SystemX telephone exchange cards I've seen with spaghetti all over the back suggests otherwise.) Designing analogue circuits seems to be a dying art these days. I had a couple of electronics students on work experience not that long ago who hadn't done anything of note with op-amps or even discrete transistors :(