I am to my dorsal-most heart muscle cell what society is to me. All my cells mostly cooperate. Certainly they cooperate long enough to build a megaproject called a human, so large-scale cooperation is possible.

But there are also lots of bacteria in the world. Way more than animal cells. And they're doing okay on average.

Indeed. Some of those bacteria would love to consume your megproject. As you soon as you lose power to resist, they get a banquet of a lifetime!