Same QA environments

I have started working on setting up test environment for End to end integration testing team. I am still a beginner.

If I have to setup an e2e environment that includes 10 applications why it is mandatory that all these 10 applications should deploy the new code on the same QA environment which will enable proper data flow end to end. That means one application from qa1 will not communicate with another application from qa2.

Could you please explain the reason behind it?