I would imagine a shop has to really know their stuff to make money assembling engines. They have to have a cut and dried system that works every time. A good machinist sets his stuff up, does the job, no mistakes, no surprises .... assembly is completely different. Can entail a ton of time consuming measurements, corrections, dis-assembly, reassembly etc. A shop might do stellar machine work, and then still just throw an engine together compared to the hobbyist that will spend countless hours making sure everything is perfect. A shop is a business .... and time is money. Just a few snags in assembly can rob the shop of it's entire profit for assembly .... so they don't have a lot of leeway for very careful, perfectionist type work ... the ones that do, charge huge, and well deserved, money for their services.
Building race engines for motorcycles ... I don't think I ever saw a shop that would put an engine together as well as a "racer/builder" ... but then I don't blame them, they couldn't stay in business do work that way.
Very tough business building entire engines .... not something I would want to do. I had plenty of people ask me to build them vintage race bike engines ... no way. My engines that performed well on the track had many, many hours in them ... could not do that for a price any sane person would want to pay.
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