Because you are connecting the batteries in parallel. If you were to connect the batteries in series you would be doubling the voltage and causing all sorts of problems unless you specifically have a 24v system.
I don't think it takes that much work. You disconnect the negative cable on the car that gets 24 volts. Connect the negative jumper cable to the post and then connect that to the positive cable of the other car. From the other car connect the jumper cable to the negative post. Connect that end to the cable you just disconnected. Not that much work. But more work than a regular jump.
It is easy to get 24V when jumping a car, just connect positive to negative with one of your leads. And if you're feeling especially smart finish the other connection with negative to positive so your battery and jumper cables promptly explode/burst into flames