Why do keyboards with passthrough usb require two usb connections at the end of the cable instead of just one?
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A USB port provides a certain amount of power.
You could have one USB plug going to a USB hub chip in the keyboard, with the USB socket and the keyboard controller both connected to that hub chip.
But in that case, you need some power for the keyboard and the USB hub chip. So there is less power for the USB socket. For example, this might mean that you can plug in a flash drive but not a USB hard disk because the USB hard disk needs more power.
There are also issues with the many many optional features that USB can support. USB keyboards tend to use USB2 or even USB1 speeds, since they’re only sending a few key presses per second at most, and USB3 chips are more expensive. USB keyboards also don’t use fast charge for obvious reasons. But you might have USB ports on your PC that support those features, and you might want to plug in a device that uses those features.
So keyboard manufacturers build a simple dumb USB extension cable, with the socket built into the keyboard and all but the last few centimetres of the cable built into the keyboard’s USB cable.
That way, all the advanced features of your USB port just work. It is much simpler and cheaper and more reliable than trying to get a USB hub chip that supports all those features. There are also no power issues. It’s much more likely to work the way the end user expects, giving happier users and less support calls or returns.
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