A former neighbour was a builder, he openly admitted if he didn't want the job he priced it ridiculously high that way he wouldn't get it but if the customer accepted the quote he would be quids in.
Always been the way, if you have loads of work and all you lads are busy you quote high as it covers the cost of subbies, if don't have much work you quote lower as you want the job and need to keep the lads in employment, Its a constant balancing act
The other thing worth noting is that you never know what you will find once you break into the structure of a house, even breaking ground for new footings can be fraught with problems, a good builder uses his experience to judge what problems there may be (and there are ALWAYS some) His price has to cover a proportion of any costs that may be incurred by problems, if there are very few on one particular build you can bet there will be a load on the next one
As for architect's I have had more intelligent shits when it comes to how buildings are actually put together
derek kelly in 30 odd years of building work I have never seen a set of plans that are right or workable without major input from the builder eg roof angles that wont work as they ridge would cover a window, stairs that wont meet building regs due to angle or available head height, the list goes on ........... and on ............. and on