r/K selection theory has been subsumed by more advanced theories over the last few decades, but many of its themes are still applicable and it provides a simple way to think about some general reproduction strategies. Herbivores fundamentally have far more energy availability, because they're a level closer to the primary input of the sun (Sun -> plants -> herbivore, energy for useful work lost at every conversion step), and in turn many of them follow a more r-selection strategy where they have lots of offspring with lower investment in each individual one, and huge populations of which significant numbers die young (from predation, accidents, starvation, disease, etc) but enough survive through quantity. Conversely this can actually make healthy adults more valuable then any single young one, since they represent a success that can and will breed many many more times.
In contrast big predators sink a lot more energy and time into each individual juvenile, and have far fewer of them. You'll notice that with many highly mobile herbivores juveniles come out and are ready to walk and begin moving with their parents/the herd in very short order without an extended period of total helplessness.
Herds do have to keep moving to find new food supplies, so staying in place indefinitely is not an option. They generally are much faster in a straight line then most predators too, so they can in fact outrun them (and in turn predators are forced to deploy tactics, go after ones with restricted mobility which takes more function to recognize, and so forth). These various pushes and pulls on selection have resulted in the successful strategies we see each species deploy. The emergent effects of life really is incredible as trite as it may sound to say it. Such incredible complexity driven by basics that are pretty straight forward.