No idea - I assume because it's easier? This isn't my wheelhouse. I just remember when I found out I was surprised it seemed nobody else knew (at least everybody else I told seemed surprised) since it's pushed so heavily as an ethical alternative to meat and they're still killing animals to make it.
Once the culture is started you don't have to kill more calves. Of course, accidents when growing cell cultures still happen.
But how much meat do you get from each culture?
It’s a peculiarity of modern American ethics that some people think that this would be significantly more ethical.