Rendering =/= output, so that's not the reason.
And it's equally stupid to think that PS5 couldn't do 1440p 120Hz when XSX can. We have seen both consoles are a match. Especially in 120 FPS modes, PS5 has performed much better, i.e., Dirt 5, COD, DMC5, Destiny, etc.
I think in the end we will likely see one of the three following reasons:
- PS4 Pro didn't support 1440p resolution, so devs just used the 1080p version on PS5 like Avengers to minimize their work and optimization.
- Sony is prioritizing frame rates over resolution and asking developers to hit a locked 60 and compromise on the resolution instead. Perhaps MS is asking devs to prioritize resolution (their marketing campaigns have been about 4K and resolution) and compromising on frame rates if needed.
- Both consoles have a dynamic resolution scaler. And both consoles will end up producing similar results, with XSX sometimes hitting >1080p resolution (because of no limits) and PS5 performing better at 60 FPS (because of the resolution cap).