Why should QA have their own QA Environment? What are the pros and cons?

If QA is using a dev environment, that environment is likely changing. It is hard to control the state of the environment with multiple people working against it.

Typically, QA environments mimic more closely production environments than do development environments. This helps ensure functionality is valid in an environment that is more production-like.

Developers tend to have lots of tools and things running in their environment that could affect QA validation.