Why can't differentiation cells be reversed?

In general, the process of cell differentiation is irreversible. However, under certain conditions, the differentiated cells are also unstable, and their gene expression patterns can also undergo reversible changes and return to their undifferentiated state. This process is called dedifferentiation

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most community's take hundreds of years to become stable

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.