Testing is not a standalone activity because:
- resolving bugs means talking with product and development to make sure that the bugs are well understood and that the path to correct them makes sense for all.
- understanding business requirements and what an application is trying to achieve means working with business users and product owners
- understanding software configuration means working with developers
- questions about what an application is trying to achieve require the product owner to help
- understanding the enviromental setup and requirements can involve devops
- understanding the issues that users are reporting that should be tested against can involve customer support
- in Agile environments (probably the majority based on some surveys I've seen), qa doesn't stand alone but has to work hand-in-hand with product and development as changes are made in an iterative fashion.
- there should a constant flow of information with the rest of the organization about what they understand the application does and what the organization wishes it to do now and in the future.