WRK is a nice tool that can bring you quite far, for sure. But what about the orchestration of complex sessions? As I understood that blog post mentioned in another comment StormForger was actually able to play a game. A simple one, but there is still a lot of complexity involved.

But even if you can build something like this with WRK (which should be possible, due to the Lua support) you still need to setup your test environment (reproducibly), collect all the logs from the machines involved, monitor the test generators, create useful metrics from the test results and make it possible to interact with them. And I'm pretty sure I forget a whole bunch of stuff that needs to be done.

And I know, that I don't want to do all this stuff by myself. With a service providing all this, I can concentrate on the real work: Interpreting numbers and fixing problems ;-)

BTW: I'm pretty sure under the hood there is some load testing tool deployed. Cannot imagine someone runs such a large scale test without such thing.