This sounds wrong to me and potentially biased. My initial guess is that milk is good food but it has some downsides. First, if the mother dies or dries up, a milk-reliant calf is in trouble, so there’s risk there. It also means the calf can’t get separated from its mother.
Also, how does the mom make milk? By eating grass then transforming it. That transformation must have an efficiency loss. I think the milk is costing resources to help the young cow – the mom is taking grass and using resources to turn it into a particularly good food for the calf. It’s temporary because it’s inefficient energy use overall so that’s only worth it when the calf is young and extra vulnerable, and also because dependence on the mom is problematic, and also because it would make no sense to have an adult female cow drinking milk from its mother while also breastfeeding, and also the parent is likely to eventually die before the child so children have to stop relying on their parents for food eventually. It’d also probably be problematic to breast feed all one’s children of many different ages at once – the mom would have to eat and process more than the total regular grass intake needs of all those kids plus herself.
Anyway some members of the species have to eat something other than milk in order to bring new energy into the species’ stomaches. Evolution had to design around milk being temporary/limited even if it’s very healthy.
Just some speculations without research.