Steam Frame

Kind of weird using AA batteries, I'd imagine something else would be better suited for this?

AA means they don’t have to handle battery replacements; and it’s not too-too hard to get rechargeable batteries.

I would prefer batteries in machine, too; but this does have some sustainability and repairability (by not needing it) advantages.

You're right that it's "not too-too hard" to get them, but it's also "not too-too easy" to actually use them in comparison just plugging a USB-C cable into the device. The process you will have to go through to recharge this will become incredibly annoying for something that will eat through batteries as quickly as a VR headset. Think of all the criticisms Apple has received over the years due to the Magic Mouses charging port being on the bottom and that only needs a charge every couple months, this will need to be charged after a few hours of use.

There are 1.5V AA li-ion batteries on the market. I bought a few to power children's toys and they have comparable capacity to alkaline batteries. At high currents they actually perform better.

Cost is about 10x that of their non-rechargeable brethren, but obviously there's return on that investment.

I wasn't denying their existence. I was comparing the process of opening your device, taking out the batteries, finding their proprietary charger or hooking each individual battery up to a USB cable depending on the specific variety of battery, and them putting them back into your device is more annoying than just plugging the device into one of the half dozen USB-C chargers we all have scattered around our homes.

I doubt this would be a dealbreaker for most people, but it's a choice that will provide a consistent small annoyance for users.