Why software companies are hiring test engineer?They can do the testing with the developers also.Then why???

I don't know but I can guess. It can be for Quality Assurance or it can be to make things easier for the developers so that the developers can spend their time in the most productive manner. If it is for QA then it is done to ensure that the code is working properly. Some software must be correct and companies cannot trust the developer to fully and accurately test code. A tester could be independent from the developer and ensure that the software is working as expected.