There's a very very big difference between a virus who has evolved alongside other living beings on this planet for millions of years and finally made the jump from one specie, vs some form of life that has probably been separated from us for billions of years. The former has very specific mechanisms that has evolved to perfectly trick our defense system, not too much to kill us instantly and not too little to not spread. It's perfectly in the goldilock zone. The latter on the other hand has never come in contact with anything anywhere close to our life form.