What would that prove? Game developers have devkits already. That Microsoft can make such a machine is not in doubt at all. The questions that remain are: (A) Can the console be made at the targeted cost? (B) Can enough be made to meet demand? (C) Does the case design allow proper cooling to ensure longevity? None of these would be answered by simply turning on a prototype. Or a production unit for that matter.
Powering on a production unit would prove that the Xbox series X has entered production,that the production boards work, and that they haven't been missing critical development milestones. It proves that AMD's APU works as expected, and are in-hand in large quantities.
There is no question that Microsoft
can make this console, but is it ready? We know they know what the design looks like, people have had hands on with dummy hardware.
What is in doubt, is if Xbox (or PS5 for that matter) will launch this year at all. Everybody thinks Sony and MS are playing chicken on announcing price and release date because they want to win on pricing... Maybe they haven't announced price/release because one or both of them don't actually yet know what it costs to build one, or when they will be able to actually get it to market. Maybe both companies are just hoping that the other will announce a launch delay first.
Sure devkits are out there - but that doesn't mean much. Pre-launch they are unlikely to have the same hardware as the production units and developers have most likely been getting hardware updates as the design has progressed for the last year or two. The first wave of devkits probably went out before AMD had even finalized Navi and Zen2. I wouldn't be surprised if the first Xbox series X devkit was literally an Xbox One X with a SATA SSD and a sticky note on the side that read "like this, but better; maybe ray tracing".
For all I know, the current dev kits could still be pretty far off from production hardware. It really begs the question why the small amount of in-game footage released so far has been 'in-engine' running off of a PC.
Anyways, usually consoles don't need to go out of the way to show something so basic as 'it exists', but usually there are events like E3 before a console launch where there are dozens of demo kiosks set up for the media to photograph and play around with. So both Sony and Xbox very much need to go out of their way to communicate that message. Surely they have the ability to put some journalists into a hotel room with a console for an hour and let them check for smoke and mirrors. If not, why not?
Is it even worth it for Microsoft to launch Xbox Series X this year with cancelled holiday shopping and no exclusive launch titles? I cannot overemphasize how close we are to launch. In the before-times, September is the timeframe you would expect working demo kiosks to start popping up at brick&mortar retailers to drive preorders.