Why we need Certificated PMP instead of experience PMP?

Well, If I have to hire a Project Manager, I would choose an experienced PM rather than certified PMP with no experience. 

As Nouman said, PMP certification just validates certain level of PM compentencies and it's not rocket science to pass PMP test. Whereas experienced PM has maturity to effectively manage projects, gains knowledge and experience to deal with difficult situations where he/she can take right steps to solve complex project issues...

The PMP certification establishes a common language among project managers and helps each other work within a common framework. Once you have the PMP, you need to consider how you're applying the processes, tools, and techniques to projects. By sharing how you apply the Project Management Body of Knowledge (PMBOK) framework and demonstrating that through actions, you'll build your own personal brand within the company as someone who can deliver. With each success, you should find experiences on larger projects across programs in different business organizations. By demonstrating greater competency managing more complex projects, you'll also be able to compete for more senior positions.

However, certification is merely a test; it validates academic competency, but it doesn't prove you have a historic track record of success. When hiring project managers, I often look for the PMP certification to screen incoming resumes and prioritize candidates for interviews, though I'm using the PMP to look for leadership behaviors across different projects.With enough study and creative experience, anyone can become a PMP, but the successful project managers have a track record of successful delivery using the PMBOK framework.

Ultimately, I believe long-term career success depends on the person and not the credentials. People, not certifications, deliver projects.