They also don't allow you to purchase books within their ecosystem. On PC, you're using Epic's payment platform. Same way you can pay for your kindle books on the web in Amazon's ecosystem and then read them on an iPad.
My point is that weird technical and legal hurdles, when followed lazily, can lead to anti-consumer practices. I don't think it's deflecting from the issue to discuss the underlying technical and legal constraints. It's Hacker News. If you want to rage against Sony for not allowing cross-play go to Polygon.