It adds to the romanticism and involvement. The original GT40 Le Mans car from the 60s that originally beat Ferrari was built from parts that you could go to your local Ford dealer and order. Maybe I'm wrong but I think that if you had enough money at the time you could actually buy all of the parts to build the same .427 they were using in the professional races. Pretty cool.
The GT40 was a bespoke race car built to settle a grudge. Henry Ford II wanted to beat Enzo Ferrari at his own game after the sale of Ferrari to Ford fell through. Ford failed to beat Ferrari in their early attempts with the GT40 and the entire car went through several revisions to reach the LeMans winning form. These were not typical parts and I would be surprised if they were widely available.
Also, domestic US engine displacements were given in cubic inches. In this case a Ford 7 liter engine is 427 (whole) cubic inches. A period is not prefixed because it is not a fractional measure.
Back then, you could order performance parts from dealerships and whole engines. Could you walk in and order a gt40 engine? I have looked and have not found a part number. However, some race engines did not have part numbers because they were aftermarket parts.
https://performanceparts.ford.com/engines/
http://www.chevrolet.com/performance/crate-engines
http://www.chevrolet.com/performance/connect-cruise-powertra... up to 727 horsepower, with transmission and electronics included and ready to go. Connect and Cruise.
Nowadays, an LS crate engine and a smallish turbo can get you 600 wheel horsepower on pump gas.
Chevy has an extensive offering of "crate engines", given equal billing with its performance car models. Ford also has a "performance parts" site selling everything from engines on down. I had no idea.
I couldn't find anything similar for Toyota or BMW; their "performance" parts catalogs seem to consist only of superficial accessories like steering wheel covers.
Not quite true. Toyota Racing Development (TRD) does sell performance accessories (superchargers) for some engines. BMW has M Performance and you could put some of the M Performance parts on a non M vehicle if you wanted to.
Other manufacturers also sell fully built race cars for customers to buy and race (Blancpain GT Race series).
Apple has been pretty lenient with the Hackintosh community -- but that may be coming to an end, if Apple moves away from intel.
If accepting that people can die if they make poor decisions, which they ought be allowed to, was a thing - I would be right there with you. But today everybody wants somebody to blame for everything, especially with new technologies, and when freedom enables people to make bad decisions that can hurt a manufacturer much more than help them.
While their primary motivation might be greed, I'm not convinced it is. Apple on the other hand...
You can get aftermarket non-OEM versions of practically every part except for the chassis for old Nissan Skylines and Silvias.
Contrast this with companies like Apple and Tesla, who attempt to maintain tight control over their products and use legal action against those who want to "open them up".
(From what I've seen, European and Asian automotive companies are somewhat less open, but once again there is still a large aftermarket.)