Our civilisations also kept slaves for millennia.

You mean western civilisation I guess? After those millennia, still 75% of the human population didn’t start or finish their milk adaptation.

Milk production is performed by making cows pregnant repeatedly (about once per year I think) throughout their lives and then killing most of their calves. Once the mother cow is worn out from all the births, they too are killed.

It's the dead calves' milk.

I'd wager most people actually do inconveniently hold some vegan ethics, but there's a blind spot due to tradition, culture, and habit.

For example, most people have some sheepishly held position against factory farming despite partaking in it. And they think it's wrong to kick a dog and a pig and a cow. And watching a documentary like Dominion or Earthlings makes them feel horrible so they'd prefer not to watch it.

It is a moral problem for many people, however changing delicious, traditional and healthy habits is not so evident. I don’t blame them.