1. The primary reason is more or less a psychological one.
Developers due to human tendency are not able to figure out the issues in the code which they themselves have developeed. On other hand a Tester is a second person who looks into the product from a different point of view.

2. Testers are considered "constructive destroyers". The destroy the product inorder to construct it in a better way.

3. Everytime developers will not get time to test the whole product along with their packed development schedule. Testers are sitting and waiting for the product to come on their desk.

4. Tester will have a breaking approach. A developer can NEVER have that approach towards the product he/she developed.