because an animal is a self-contained unit which manages disease and feed conversion that scales fairly predictably, while cultured meat has a pretty high entry barrier in terms of feedstock, sanitation etc
The vat based solution is more scalable in the long run, and not impossible (we do it with beer).
I have an old book somewhere that says the milk cow is the most efficient means of converting a rocky hillside covered with grass into human-usable protein.
Yeah, that's scalable.
reference: http://www.nytimes.com/2002/03/31/magazine/power-steer.html