While I am not a developer on PSO2, I am a software developer who's worked with multiple devices and porting. I'd like to share my opinions on why I feel a PC beta is unnecessary.
The basic gist of it is that PSO2's never been on Xbox, and the US launch is the very first Xbox One port, which means there's a lot that needs testing. While Xbox and PC run Windows, the versions they run are separate codebases. Also, the hardware factors in heavily. While PCs all follow a similar build method, meaning modularity can be accounted for despite varying configurations, game consoles are built in a different manner than PCs and use different methods of graphics rendering, security, and more. Porting isn't as simple as pushing a "Port To Xbox" button, a lot of work goes into recoding and remaking graphics, as well as bug testing, fixing, etc. I found this out in my attempts to port a Unity app from PC to Microsoft's HoloLens system. (Even the different HoloLens models run on different chip architecture!)
But then why have a PC beta at all, if the PC version is done? Simple: it's not the same code as the original on PC, either. This is a localization, not a simple retranslation. YouTuber Scott the Woz has a video on localization and what it entails, from removing questionable content to even complete mechanical changes, such as the Mission Pass. Even minor translations require a lot of work, as my good friends on the fan translation team Precise Museum found out when we had to not only translate text, but make sure no overflow errors happened (such as in Kirby's Ghost Trap/Avalanche), replace images (believe me, it was painful), and do bug testing for that.
Here's an example of overflow, using the Kirby's Avalanche example I mentioned (source: The Cutting Room Floor):
The localization changes that PSO2 needed to go through were likely text translation, voice acting, addition of new US content such as the Game Pass Ultimate content, removal of JP content that was questionable or overused, and the addition of a brand-new battle pass system that likely required quite a bit of work to implement. There's also the fact that the US beta is in... well, the US. The game's data had to be hosted on a US server, which Microsoft graciously provided in the form of Azure, meaning the game's code had to be rewritten to link to the US servers instead of JP, which... ...well, it takes a lot of time for a lot of content.
This is pretty much how I imagine it. There could be more or less to it than I thought, but the fact is that Xbox absolutely needs a beta. PC could use one too, but right now, it's the Xbox version that needs to be checked for bugs in the porting process.