Huh. Well that's odd. Is that a long term thing? Usually mergers and acquisitions actually merge executive oversight between two companies don't they?
Keeping in mind we don't have access to any private long term plans or anything, but it seems that it will be ongoing for now. Partially this seems to be related to their limited integration policy for acquisitions, and partly a practical matter so as not to overload the existing management.
If you want to see the ultimate example of a limited intigration gaming acquisition katamari of doom, see the Embracer Group, whose subsidiaries pursue independent mergers and acquisitions, and which runs about 6 different publishers each with their own duplicate staff for various duties. And some branches publish games from other wings of the company! Letting your acquisitions stay intact means inefficiencies from duplicate staff, but it's also attractive for the companies being acquired, and is less likely to mess with what they're already doing - I.e. Better to be a bit inefficient than to screw up a good thing. It also let's you just hit the ground running because there is a bare minimum of time necessary to "absorb" them.
As far as Microsoft's gaming structure, Matt Booty runs XGS, Todd Vaughn runs Bethesda, and both people then report to Phil Spencer who runs Microsoft Gaming.
I'm the future, it's very likely that this will be joined by a 3rd person running Activision-Blizzard-King, also reporting to Phil directly. There is also a chance they might choose to abolish that joint structure and have Blizzard and Activision and King as 3 separate lines with a direct interface to Microsoft, but it is above my pay grade to comment on the likelihood.
I suspect that the next showcase in 2023 post merger will not be called "The Xbox + Bethesda + Activision Blizzard King showcase". At that point they will probably call it something like "The Xbox Family Showcase" but still keep the sub-brands separate during the show and in marketing materials. I also suspect that future games from Blizzard will display the Xbox and Blizzard logo side by side on the publisher splash screen. This is comparable to a service like Disney+ where they want you to know what is Disney, what is Lucas, what is Marvel Studios and what Is Fox. They all have brand recognition, prestige, etc.