Mobile maybe but their PC base for Fortnite is supposedly massive if the original PR for EGS is accurate. It is also the place where all transactions are free of platform holder fees.I think it's plain and clear that Valve is heavily invested in ensuring that the PC remains a healthy and open gaming platform for the foreseeable future. Their entire business model revolves around it.
Epic, on the other hand, has a much broader focus. I suspect that they make the majority of their revenue from consoles and mobile now. So I am not entirely sure what their long term goal for EGS is (or if one even exists). Selling a small number of heavily curated AA indie games at a 12% cut doesn't sound very profitable compared to their other ventures.
Not really surprising why they were the ones spearheading cross platform play when you consider that. Anyone they can get to buy on PC to then play on PS4/Xbox/Switch are helping Epic circumvent the first party royalty.
I seem to recall some comment that was made/leaked from them recently that the 12% thing wasn't even remotely sustainable. I'd imagine they're just trying to carve out enough of a presence to make a permanent place for themselves.
Probably my biggest concern would be them getting to that point, pulling back on the overt paid exclusives, but replacing it with a modified UE licensing scheme that effectively encourages the same end.