Because just outside the voting booth there are actual people watching after your safety and making sure you aren't coming into the booth with a gun on your hea

Because just outside the voting booth there are actual people watching after your safety and making sure you aren't coming into the booth with a gun on your head or any other kinds of means of coercion. Also, each candidates can make sure the others aren't messing up with the process. A bank has no way to know in real time if you are being forced to transfer hundred of thousands of dollars or not.