Because the injections will cause the animals to produce the spike proteins, which will not only trigger an immune response in those animals, but, when we eat this meat, the spike proteins remaining in the animal's blood and tissue will enter our bloodstreams. This will trigger the same immune response as having taken the vaccine (although it won't alter our DNA or permanently program our cells via mRNA insertion). Whenever we eat such meat we will experience some manner of similar symptoms as those who have taken the vaccine, such as blood clots, etc.

An example of this same mechanism at work, but with a different class of proteins, can be seen in those with celiac disease; the gluten proteins enter the bloodstream, and antibodies attack that protein, causing systemic inflammation.