Intensive farming, technology (machines, antibiotics), breeding of extreme animals, cheap food and water and land for animals, subsidies.

https://en.wikipedia.org/wiki/Agricultural_subsidy

https://www.youtube.com/watch?v=KcA3P6abmjY&t=817

https://www.youtube.com/watch?v=Zctdaxvewng

Intensive farming is a big part of it. Chicken meat used to be fairly expensive until after WWII, when the Americans invented factory farming. Mechanisation is the other side of the equation.

We require less workers per hectare for agriculture and horticulture, it's been going down steadily since the industrial revolution. At the same time, intensive farming techniques have been developed. Chicken used to be a fairly expensive meat until after WWII when the Americans invented factory farming.