Why did unicellular organisms not die out, but survived to this day?

The goal of any species is survival and procreation. Unicellular organisms are no exception. The ability to live for billions of years these microorganisms gave simplicity and adaptability to the environment. Bacteria and microbes survive better than other animal species. If during ice ages, warming, changing living conditions, falling meteorites, most living organisms died, bacteria and microbes felt great. Moreover, almost all microorganisms that are found now were millions of years ago.