I don’t know how those bugs get there in the first place.

Easy to explain.

Visualizing program code as cables, this is how a shiny new program would look:

And this how, for example in a commercial environment, or if the programmers are not absolute masters, such a program will look after 15 years of bug fixing, feature requests, urgent changes, deadlines, and unfinished restructurings:

The thing is that especially C code contains tons of implicit invariants which you simply cannot maintain in such code.

And don't forgot that the picture above shows perhaps 500 cables, but a codebase can easily contain 50000 lines of code....