Why Developers are not good testers?

Misunderstandings will not be detected, because the checker will assume that what
the other individual heard from him was correct.

Improper use of the development process may not be detected because the
individual may not understand the process.

The individual may be “blinded” into accepting erroneous system specifications
and coding because he falls into the same trap during testing that led to the
introduction of the defect in the first place.

Information services people are optimistic in their ability to do defect-free work
and thus sometimes underestimate the need for extensive testing.

Without a formal division between development and test, an individual may be
tempted to improve the system structure and documentation, rather than allocate
that time and effort to the test.