I have built hundreds of PCs -- I used to work at a company doing this and investigating the various integration problems in a specific configuration. Anybody can slap parts together. The problem is achieving the final level of integration and reliability. You can get lucky, use a *supposedly* well-known configuration and it might work. Or maybe one of the components has undergone a running change in manufacturing, microcode update, etc. which leads down a rabbit hole of trial-and-error debugging, returning parts, trying new ones, more tests, etc.
Nowadays it can often work well but there is always the risk of hitting one of these problems which can be very frustrating and time consuming. That is why large PC manufacturers have integration and test departments. Some engineer is sitting in front of a logic analyzer studying an infrequent, transient glitch caused by an incompatibility between components:
https://joema.smugmug.com/Computers/Tektronix-TekMSO72004/n-6z8VhG/i-52kXRLG/XL
They send trace files to their counterparts at other companies trying to figure out whose component is responsible for some difficult-to-reproduce problem.
A home PC builder cannot do this. They must simply hope that the individual components work perfectly and they don't get caught in a tedious, infrequent integration problem that saps time and patience. Usually it works pretty well, in fact I am typing this on a PC I built. However despite my extensive experience I am tired of doing that and do most of my professional video editing on an iMac.