"Who said developers don't test? They perform unit tests on the code they develop.
However, if your question had said, "Why developers should not perform System or System Integration Testing", then the answer is simple:…"
Developers are writing the code and if same person will test his/her code then he/she will not get the issue (more than 70% cases).
Thought process is different from the tester.
Tester has more time to break the running…"