Maybe because you can’t manifest extra dairy cows out of the aether in the winter?
No but what you can do is use dried milk fat to even out production, you can do is simply pay ranchers more for that month given that they are making less, you can do is freeze butter a month earlier and release it when milk gets more dear, what they can do is subsidize hay so it can be introduced slowly creating less of a shock.
We don’t do any of those things. And why should we? Dairy lobby is no where near as powerful since it is spread out instead of concentrated. Which means less lobbying, less bribes, less government money.