There's two kinds of people in this world. Those who think everything they don't know is rocket science and those who know nothing is really rocket science.
You know there was a time when I would have wholeheartedly agreed with you on that. Like my dad would say when I was a kid "Yankee ingenuity can do anything".
I've made a pretty good living doing things the majority of people think is rocket science, but I've also been humbled a lot while learning how to machine stuff, design stuff and run a business. A lot more than I ever thought I would be. I don't think there's two kinds of people. I think there's a broad spectrum and each situation is unique.
I do not believe there's much about combustion engines that is really complicated. I spent the majority of the time I was supposed to be in classes during my Junior and senior years of High School scrubbing parts out of the hot tank for no pay, just the privilege really of hanging out and listening, learning in one of the best race engine shops in the Portland area. Like Friday night dirt track each class would have engines that came through that shop 1-2-3. The local gear shop owner knew that a 600HP BBC from that shop was going to destroy axles and other parts that any other builder's "1000 HP" big block would be fine with.
What I took away from that experience is that building an engine starts with a plan and requires incredible attention to detail and the experience to know where to focus that attention.
Sure you can probably figure all this stuff out, but you gotta have some serious, expensive machines to do better than the guys charging $20 a hole. So why?
CNC hones start at $50k last time I looked. I would buy a really high end used Japanese VMC with scales and 60" of X travel just because I will believe it when a Kuraki or Mori tells me a position is right on or that shit is straight. And I can probably find a nice 1990's one for less than that shitty 7500 lb engine builder specific turd thing.
Then you need a ton of other stuff. What else do you want to do? Main line work? Crankshaft mods and grinding? Head work?
I think a guy could piss away a quarter mil real quick just for the excitement of handling 325 lb blocks and fixing holes for $20 a pop for cheap fucks who complain about the prices.