Writing good tests is hard: test code tends to be opaque, fragile, and hard to maintain even when written by the same developers who write decent product code. Why?
Test code is inherently introspective: it is about the code it is testing. Product code is about a user-domain problem - for example, managing the relationships between a company and its customers. Test code is about the user-domain problem and about the product code. As a concrete example: writing tests often requires exposing (through package sharing or through "test-only" APIs) methods that don't make sense otherwise, in order to give the test code the ability to inspect the internal state of objects in the product code.
Introspection, or self-reference, increases conceptual complexity. An object that does something is simpler than an object that reasons about an object that does something.