Offering what was considered valuable
4:4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
Again, where does it say they ate the animals? This would be an assumption. Sheep offer wool for makeing clothes, rugs, etc. and milk that can be made into cheeses.
The offering was not based on offering something of value otherwise Cain’s offering would have been o’kay. The offering was based on what God had determined was the correct offering...it comes down to obeying God.