Man, nobody here with the answers.

Selective breeding for the last 10k years.

A heifer is an unbred female cow. Heifers do not produce milk. A dairy heifer is going to be a cow of a dairy breed, Jersey, Gurnsey, Holstein, there's a lot. Beef heifers are never the same breed as a dairy heifer even though they're functionally the same at that life stage.

Once a cow is bred it'll go in one of two directions. The beef cows only produce enough milk for their calves but all mammals produce milk with a supply/demand method (generally). The more milk a calf drinks, the more milk a cow produces. So until weaning begins supply slowly increases with the calf's appetite. For some things you might be able to get some excess milk even from a beef cow but nowhere near the quantities you get from a dairy cow.

When dairy cows have their calf they do generally get promptly removed from momma. But momma is still useless for milking at this point. She's not producing milk. She's producing colostrum. Colostrum is discarded by some farmers, milked and fed to the calf by other farmers. Either way it has to be milked from the cow. Milk production starts quickly, though, as colostrum is only ever at temporary thing. The calf is always removed because cows are on the farm to work and where a beef cow's job is to make meat (which they can do by making more cows) a dairy cow's job is to produce milk.

A dairy cow that has already had at least one calf and is due to have another will actually go dry shortly before she has the calf. Her glands are switching back to colostrum mode.

Dairy cows produce so much milk that their udders are gigantic compared to a beef cow's, even when both are lactating. They produce so much more milk than any single calf could ever drink. They also don't necessarily produce the sort of milk a farmer wants to feed to the calves. Calves drink fortified reconstituted milk (made from dried milk that's had a blend of nutrients added).

The reason why a cow's milk and their calf's needs don't necessarily line up is because of the selective breeding bit. We want cow's milk to taste a certain way. But we also want calves to get a certain sort of nutrition. The nutritional needs of a calf, particularly one you're raising for a specific purpose, is not going to give you the stereotypical flavor of milk, cheese, and yogurt you get in the store.

You take away the human element and yes, a dairy cow and calf could easily stay together and we'd still have plenty of milk. But there's no profit in producing milk. Farmers make pennies per gallon so in order to stay solvent they have to maximize production and quality. So the only time you're going to see a farm where they can keep the calves with dairy cow mommas is a farm that likely packs and sells the product themselves rather than selling it to a factory or brand.

Sauce: I grew up in a farming community for 20 years and visited dozens of mom and pop farms providing cow healthcare and saw the whole industry as seen through the eyes of farmers with less than 10,000 cows.