General Question : Why do so many SoapUI users appear to use Groovy scripts to run tests?

I think many answers are misunderstanding your question as I understand it.
I read your question as why are users using Groovy to run test cases.
I'm aware of two situations, one is to use a previous test case as a setup for the current test case. The second is using a test case as a library call - this is when the library procedure needs to make a HTTP  request to the system under test and these requests are not done via Groovy.