That's because in older generations, milk was framed as a stupid children's drink for children, so teens and adults didn't drink it. After all, a cool 17-year-old doesn't want to look like a BABY in front of her even cooler friends! This was especially true for girls and women of the time, who might also stop drinking milk in an attempt to lose weight.
As a result, practically an entire generation developed osteoporosis and other health problems due to a lifelong lack of Vitamin D and calcium. Ask any woman you know over 50 and she'll probably have a story like "oh yeah no one drank milk and I really regret it because a light breeze will break all my bones." For one example, my mother-in-law has a calcium deficiency that is affecting her health. But because she hasn't drank milk in like 60 years, it's both hard to get in the habit of having a glass, plus the texture is kind of gross to her since she's not used to it. Which in turn makes it harder to fix the deficiency.
So go back to the early 90s, and combine public health officials desperate to prevent another generation of vitamin-deprived kids, 90s parents looking for alternatives to soda to give their kids, and dairy farmers around the US concerned with their low sales, and you get the Got Milk campaign.
(Please note this is just me giving historical context to weird retro ads. I'm aware that there's other ways to get nutrients in your diet, and I'm not saying anyone HAS to drink cow milk specifically)