Definitely not a symptom of Linux being a hodgepodge of code thrown together from a thousand different sources and no one person could tell you how it all fits.
Of course it's (indirectly) a symptom of that.
What's the alternative? Proprietary closed-source operating systems owned by corps who can be compelled to insert covert backdoors?
If BSD was as popular as Linux it would have the exact same problems.
I wonder if you think other OSes are any different?
TempleOS is the only thing that comes to mind that doesn't fit your description and it's not practically useful.
Any sufficiently large codebase is a mix of ideas and concepts implemented by different people with different priorities over a large timespan and if you can fit the entire thing in your head it's not very interesting or complex.
"Mythos, find me a bug in LUKS. I know there is one in there".
Bugs happen in all code. The difference is, anyone can fix stuff in open source. Closed source bugs are out of control and must be worked around. Usually by switching to OSS