The basic problem is that many systems grow organically as new features are required, but as new features are added to the system it becomes more complex and tightly coupled. Another aspect I have noticed might be the 'Black Hole' syndrome. In systems that are custom written and business critical there is no clear scope of the project. With no clear scope anything new that the business needs is simply thrown into the existing system. The never ending scope creep means that the system takes more and more responsibility. And so grows a monolithic tightly coupled black hole. Because it is at the centre it tends to attract any new requirements because anything new needs to interact with it. And the more you add the harder it is and the more bugs you introduce.
Is this the manpage for systemd?