It comes from early days of political debates, before television and radio. The politicians would have to go to the voters and gather in town halls or churches to have debates in person. But due to scheduling there would often only be one politician showing up to these meetings. So they would literally build a dummy out of cheap materials, usually straw, to hold the debate. As you might imagine this was kind of silly with one person making up the arguments for both. That usually meant that the strawmans arguments were arguments that was easy to argue against and often quite silly. Usually just a faint resemblance of what the opposing political party actually meant.

You still hear this done today because it is a very effective rhetorical tool. If you are controlling the medium you can make up any silly argument for your opponent to make your side sound more sane and valid.