They eventually do, but it can take several weeks and a noticeable amount of lost production and hence income. Growing up on a dairy farm, switching back and forth sucked for the cows and the rest of the livestock.
It's a common myth that farmers want daylight savings, but farmers are actually usually quite opposed to it. Daylight savings was originally proposed as a way to conserve energy, but it wasn't put into practice until the world wars, where energy conservation was actually important.