If you are asking why don't we make pet food that is plant-based, well, we already do. There are a number of vegan and vegetarian dog foods on the market already. Just ask your vet.
If you are asking if Beyond Meat is like plant-based dog food. It does share two of the same ingredients. the Impossible Burger has even more ingredients in common, about 10 out of the 40 ingredients in an IB are also in the best plant-based dog food.
Yes, plant-based meats are highly processed foods. And are not as healthy as whole plant-based foods. They are probably just a little healthier than their real meat counterparts. However, they are way more environmentally friendly. Plus the whole thing about not having to kill any cows, chickens, or pigs to make them.
Dogs are not carnivores. They are omnivores just like us. One theory is that they evolved to be omnivores after living with us for 20 or 30 thousand years. but the present theory is that they have always been omnivores.
A carnivore can only eat meat. They can not digest carbohydrates. they lack the enzymes. Meat has no carbs to speak of.
A herbivore can't digest protein. They don't have the enzymes. Instead, they can digest carbs. the ruminants (like cows) can even digest cellulose - which non-ruminants cannot. Not all herbivores are ruminants. Rabbits are a good example.
An omnivore can digest everything except cellulose (fiber). Pigs, dogs, bears, rodents, and people are all omnivores. Plant-based dog food has added vitamins, minerals, and amino acids so that the dog can be healthy eating it. The Impossible Burger has added vitamins, minerals, and amino acids so that the person can be healthy for eating it.