But I thought PS5 was better than a PC? Why do they need to show it on a PC?
Because it's where game are developed and tested in the earlier periods of game development, before starting to use devkits and test kits.
Specially in launch window games, being developed in a big portion of their development without having a prototype devkit available, and even without knowing the final specs of the console.
When the devkit is finally available, there is a time needed to port the game to the devkit and to optimize it, and a big part of that is made during the last months of development. Which means that as of now, devs may or may not have the game running on the devkit, but it's more likely to don't have it optimized in PS5 and may be easier to run it with better performance in a PC that may be even have a less powerful hardware than the console itself.
In addition to this, many marketing teams don't have access to a devkit, and/or is more confortable for them to be able to capture video in the same PC they are using to edit the video, add some texts, etc.
This is the reason of why footage of even many console or smartphone exclusive games is captured on PC, and why many people sometimes thinks there's a downgrade or stuff like that.
Many months, or even years before release, devs don't know exactly final games will look on each device because during the development of all games there are many changes, fixes, improvements and sometimes stuff that needs to be dropped because it's causing some issue with something else (frequently causing performance drop somewhere else in the game, or that causes a bug when mixed with something else, or it's an effect that they though it was going to performe well in that version but later had to remove or reduce it because it wasn't the case).
There are no dev/marketing team conspiracies there to deceive players. They are humans who often aren't capable to predict how finished games will look in the future.
We're certainly not going to notice the difference on a Youtube stream that's been compressed to hell.
I think YouTube really sucks with image quality, and Twitch even more. Let's hope game companies start using another place, at least something complementary to YouTube and Twitch to upload their videos in lossless (or almost) quality.
Because people even in videos uploaded to youTube after the stream can't see all the detail, and this generation we'll see a lot of change in these smaller details.