Why aren't they subpoena-ing Sony or Microsoft? Now that'd be fun for the whole family!
Those three are even less likely to want to provide the information Apple wants, especially since they likely have bespoke deals with Epic and other developers/publishers. I dunno why Apple's lawyers targeted Valve specifically, but maybe they figured there's no love lost between Valve and Epic right now and that Valve would be more forthcoming with something they could use to slap Epic down.
From what Valve is saying, Apple should ask the publishers/developers of the 600 games for the data rather than being compelled to divulge it themselves. Seems reasonable to me since Valve doesn't even have a horse in this race, but then I'm not an antitrust lawyer so who knows what will come of this.
What I will say is if Valve is forced to disclose confidential business deals such as sales and revenue share in court, there are going to be a lot of extremely pissed off third parties. And while what Apple is doing is bullshit and shows a pattern of troubling behaviour (a Silicon Valley giant dragging everyone they possibly can into their dispute to toxify the whole thing - see their attempt at blocking Unreal Engine from Apple's ecosystem, and I don't think we've seen the end of that just yet), ultimately this has all come about because Epic decided to throw a Fortnite PR stunt.
The dirtier this gets, the more the games industry will come to loathe both companies.