They’re much lower cost than satellites, and if you think you kind find yourself in a war where access to space and/or durable space assets might be denied, good to have an agile backup capability. Might as well just run it to gain expertise and tech.
It might be true that a mass produced balloon would be cheaper than the Sidewinder missiles being used to shoot them down currently, but that is because the missiles are way overkill for taking out a large, slow moving, easy to detect target. A dedicated missile could be way cheaper, whereas large balloons will always be quite expensive since you have to buy the helium.
