Cost to much to do all that sfx
Even if they did show every kill, there were still way to many. They were going to get overran eventually.
When you're fighting an enemy combatant that can fire at you with machine guns and explosive rounds, then you can get overrun. These aren't 28 days later type zombies. They're just malnourished, dehydrated, people infected with a communicable disease. That's all there is to it.
Say there were a thousand of them trying to enter the compound, the 50 cal should easily take down the first 100 of them in less than 20 seconds. Those rounds turn human flesh into paper machete. So now what affect does a pile of 100 dead bodies have on the invasion route of the zombies? They'd have to go over them or around them right? Within 5 minutes, there would be a bottleneck, thus making the capture of the base extremely difficult.
Essentially shows like the walking dead turn physics, strategy, and tactics, into fantasy, and fantasy into reality. Reality is cool too! Reality can work. It took a long time for space odyssey's to come to grips with this idea that you can create great science fiction while still observing the most basic laws of science. The zombie genre has yet to embrace this idea. For example, every blockbuster space film nowadays has a NASA scientist on set / in the writing room to help flesh out the details with the understanding that some laws have to be "bent" a little to create a good story. When is the last time we've seen a zombie film/show where they were like "Oh man, we had doctors from WHO and the CDC on set helping us every day. We had retired senior military officials on set helping us craft the military response. Some of those paramedics you see down there are actually real paramedics, showing us what to do in the event of a mass casualty incident. This story is real man. The stakeholder reaction is real. The panic that we show is real. The zombies are fake."
Anybody can write a pretend script about landing on Mars. Anybody can write a pretend script about the zombie apocalypse.