Labs don't report how many animals they kill, so no accurate numbers are available. But based on the number of mice I put down on an average day (about 150 to 200), and the number of comparable labs in the country, I would estimate that U.S. labs kill upwards of 16 million mice per year (the Humane Society estimates 25 million). For scale, that's about 40 football fields packed tight with a layer of mouse corpses.
Or enough to lower the global happiness index a couple of points, if you prefer.
So why do we do this? Animals are "culled" because they are the wrong sex, are surplus to requirements, or don't have the right genome. Animals whose ID card is lost or damaged become useless for research because they cannot be identified; they are culled. Animals which have survived experiments cannot be used in another experiment because the first experiment may affect how they respond to the second -- these, too, are culled. Animals who show signs of stress may be culled "to put them out of their misery." If it's not financially feasible to treat an animal that gets a disease, then it's say hello to the guillotine.
"This one can't tell a good knock-knock joke."
"Maybe he can get some pointers from God. Introduce them."
And, yes, sometimes death comes to animals for really stupid reasons. One time, one of my labs ordered some adult female rabbits and was surprised to find that one had given birth to a litter of baby bunnies while in transit. The researcher had no use for them (and had not applied for permission to have any extra bunnies), and the breeder could not take them back as they had been exposed to pathogens outside the breeding facility. So the veterinarian on duty euthanized them. He cried while doing so. Everyone in the room cried.
You might be asking, "Why not just give the non-diseased ones away as pets?" Well, many of them are copyrighted by the lab that bred them, with researchers signing agreements not to let any of them out of their control. Others are immunosuppressed or otherwise genetically damaged so badly that they couldn't survive outside a lab. Also, most of these are not popular pet species. Where could you possibly find enough homes for 30,000 mice? You would need 3,000 very big snakes.