Why Requirements Management is necessary?

Often I have seen Project Managers skipping on the important part of Requirements Management and considering it as a low priority job. But they forgot the principle of "Garbage In is Garbage Out". The basic fundamental of any system is

1.Input
2.Processing
3.Output
If the very first step is wrong then the consequent steps will always be wrong. No matter how much is the process defined and whether you are follwing the world's best practices.

Many Project Managers shy away from having a buy in from various stakeholders,especially the Accounts Team, regarding the Requirements Management. The reason - Many of them think that they know it all and why we should waste time on Requirements Gathering. The very famous quote is " We have done it before". But every project is unique and is a temporary endeavor to create a unique product, service or result. You can very well analyze and compare things from the previous closing of various projects but input needs to be well defined for the new project. The Project Managers starts acting like a Operations Manager.

I appreciate and understand that having a buy-in is quite difficult when you are working globally and with people of different culture but the fact is you have been assigned the role of Project Manager to get this buy-in.