Engines usually push (their contemporary) materials to their limits so it's not really a surprise that they're harder to make from scratch. The infrastructure to make ICEs is one of the largest in the world and they are only so cheap because of their scale and production steadily getting automated since the post war consumer boom.

Making that automation, however, has been easier than ever! DIY CNC designs are a dime dozen ranging from routers to pick and places to mills, there are at least half a dozen serious open source competitors for the firmware to drive them, and the welding equipment and skills necessary to make a CNC rigid enough to cut steel is more accessible than ever (designs exist even!). All the parts you need for all kinds of mechanical contraptions and automation buildouts are available from McMaster Carr with same/next day shipping in some regions and there are even low cost open source robotic arms with six degrees of freedom! Something as simple as aluminum extrusions have created a sea change in how mechanical structures are built and modified from cells in automated factories to 3d printers to automated labs. Hell, you can make 3d printed liquid cooled rocket engine nozzle in metal by uploading a CAD file and test it in Mojave a few weeks later without ever touching manufacturing equipment. /rant