If multicellularity has advantages why are most living things still unicellular?

Question

I am not sure, but is it because that they can double themselves faster?

Answer 1

Each life form adapts to it's own unique environment and has advantages within it's environment.

It's called an ecological niche. We cannot consume the tiny particles that single celled organisms live off of, and we can't divide ourselves every eight hours. So, they have that advantage.

We don't have to stay in a puddle and hope it doesn't dry up. That's our advantage.

Answer 2

Because being unicellular has advantages, too - faster reproduction, easier to survive in lean times, easier to survive in multiple environments, et cetera.