This is simply not true. What you probably mean is it is not possible to achieve the same results while not caring how it is done (ie. going about our lives).
Throughout the history people relied on dairy way more than today. In some places it was almost life or death as during winter grain and dairy was the only feed available.
Of course, today, you don't want to spend your entire day tending cows and the field.
But don't try to imply that our way of life is the only one possible.
There exists whole industry of keeping cows and producing diary in humane way. It costs more, true, but not nearly as much as to make it impractical.
It is simply our choice -- people choose the cheapest product the cows be damned.