In ancient times livestock was precious and more than a source of food (wool, milk, etc) and sacrificing one was usually a great expense. Livestock was worth more alive than dead, so were not a source of meat except in hard times or when the animal died. Wildlife was the usual source of meat, at least for some tribes. Animal sacrifices showed God you were willing to go without for his sake. Thithing is how we do it today. That and donating time to charitable endeavors, etc.