Why do companies require so many different test from one place to the next if your always doing the same kind of work?
Certain tests are required to qualify you for the correct material, process, thickness and position. The code you're welding to will specify the test requirements. Often, these tests can look far different than the actual welding you'll do in production or on the job site.
Each employer is responsible for their Welders qualifications, so if you hop from employer to employer each of those employers are required to give you the required tests. The employer is held liable for your qualifications, so they would not typically accept a qualification from another employer.