It’s because when you’re in the middle of things with a lot of context in your head, adding another little wrinkle feels like a negligible complication (in particular if the new wrinkle is ostensibly to reduce some complexity, or to ship more quickly), but those complications accumulate up to the limits of any developer’s comprehension (and beyond) rather sooner than later. Hence developers tend to work close to the limit of what they can handle in terms of complexity, which for anyone who hasn’t all the same context in their head (like the developers themselves some time later) is really more than they can handle in an effective manner.

From another perspective, this is a form of entropy: There are many more ways to increase complexity than to reduce it. This is also the reason why biological life has been getting more complex over time, the only criterion being if it’s fit to survive and reproduce.

The commonly used term, also mentioned by Fred Brooks, is accidental complexity. Accidental highlights the non intentional nature of devs introducing complexity.