Cells (as in the bit that makes it a contained volume) are both a thing that forms spontaneously without biology, and very useful for isolating one blob of chemistry from others right next to it.
There is however life on earth with huge cells and multiple nucleuses: https://en.wikipedia.org/wiki/Plasmodium_(life_cycle)
But also, the Viking test was more general than "cells", it would have ben positive with anything that could, to simplify, eat the food it was given.
More precisely, they are just cell membranes, not cells. Cell membranes form spontaneously from many fatty substances, the root cause being that oil and water do not mix, i.e. fatty substances are hydrophobic.
A dead cell or cell membrane does not exchange chemical substances with the environment (except passively, i.e. when molecules or ions diffuse from high concentration towards low concentration).
A living cell has not only a cell membrane that separates the interior from the exterior, but it also has molecular pumps and ionic pumps embedded in the membrane, which selectively pump some substances in and other substances out. Without this controlled exchange between interior and exterior, the growth and multiplication that define life are impossible.
What is characteristic for life are the molecular pumps and ionic pumps. The cell membranes have only the passive role of preventing the substances that have been pumped in one direction to come back to the place from where they had been taken by the pump.