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.