This has nothing to do with policing. Industries create standards all of the time to help the industry as a whole to appeal to their customers.
For example there are standards for threaded fasteners. The reason is because it it makes the whole concept of interchangeable threaded fasteners more appealing to customers. If I had to stick to one vendor who made their own size for everything I might just use some other technology. Because they got together and came up with a system that allows for a few sizes that meet almost all needs makes it almost trivial to select them.
The same with rating agencies. There was a public outcry for a while where parents (who actually pay for the games for kids) to provide a way to determine what content games contained. It is in the industries advantage to make a clear system for these parents. They can spend more freely knowing that a quick glance at the box will tell them what the games content is. If they make mistakes and mislabel content parents would be less likely to spend money and might take the time to look up reviews which would hurt sales.