I believe it is because most indoor cameras are powered from a USB brick. They are dirt cheap and readily available, most low power electronics seem to be powered by those now. Most indoor only cameras are also consumer grade, and consumers understand the USB brick system. However, for outdoor use, USB is not so convenient. Most current outdoor cameras are powered by POE, which is more like 40 volts or something. I'm not an expert on POE, POE+, etc. Most cameras appear to be able to powered by 12 V separate feed, but I'm not sure why since most require ethernet, may as well power them with a POE switch or injector, rather than a separate wall wart.
My guess is that 12V is better voltage to transport power over longer distance than 5V because of voltage drop, there is no current drop over distance, only voltage so to transport 10W of power you'd better use 12V/0.5A than 5V/2A and internally the cameras do not really work at 12V so even if you get 9V at the end it should be OK which will not be if you drop from 5V to 2V
PoE is 48VDC to keep the current requirements low enough for a 100 meter cable length. It is regulated down, buck converter, at the camera to 12VDC then the camera electronics take over to make that 12 volts whatever each particular circuit may need. Given that security surveillance is a 24/7 task for a camera PoE makes sense. The 12VDC input is handy for testing or if the PoE circuits fail, kind of a backup way to supply power if needed. I don't think you'll find a 2K or 4K USB camera with a big enough sensor and IR illuminators in the first place. The power demands are far too high for a USB wall wart.
Additionally, brick style, or plug and pray especially WiFi, cameras are not a good solution. Every surveillance camera needs to be tuned for the best image because every location is different and has different lighting. Leaving a good camera on "auto" settings usually results in failure at the worst possible time, IE when you need to see who did what.
USB cables, connectors, and power bricks are not weatherproof, not even close. There's a hard maximum length for a USB cable, too, its like 21 feet or something. none of this is conducive to outdoor cameras, mine are all on 75 foot, 100, and 150 foot cable runs.
the only way I'd consider doing rechargeable battery powered cameras would be with solar panels, and that would probably be a 12V system, either using AGM or LiFePo4 12V batts.
but now USB C PD includes profiles for 18 watts and even higher. my wife had a 90W laptop from her $job that ran off USB C port with the right 90W adapter and port expander..
