The keyboard will connect to the computer, so can’t the mouse piggy back on that wire by plugging in through the keyboard?
Edit: Turns out I just never knew they exist all over the place. Thanks! Learning a lot of cool new keyboard features out there.
I think the more interesting question is why you wouldn’t want it, and why it’s a standard. Which I think has to do with flexibility, honestly. I use my keyboard on a different surface than my mouse often, and you need your mouse wire to have a surprising amount of lack and a consistent direction towards the back of your table, otherwise you get weird pulls on it and it’s more annoying to use.
The reason is probably that it costs slightly more to add an extra USB hub controller to a keyboard. Margins aren’t that great when it comes to keyboards (at the low end of the market), so unless you’re Apple you’re going to try to be cheaper than the competition.
If that were true you’d have started to see more USB passthrough for wired mice in keyboards in the past decade when keyboards went from the 30 bucks throwaway that comes with your computer and you use until it melts and into a big techie status symbol. And you probably would have seen charge USB ports already built into wireless keyboards repurposed for mouse passthrough. Neither is the case.
Instead, I’d argue that a) the erogonomics of your mouse going to your keyboard are bad, and b) wireless mice are the practical next convenient step, as opposed to daisy chaining your wired peripherals.