Things can often be made more efficiently in larger quantities.
People who do the same task repeatedly tend to do it faster that those who only do it occasionally. Hence it is more efficient to divide tasks by people instead of being self sufficient.
Building a machine to do a repetitive task may take time, but if use of the machine can then do the task much faster, then economies of scale are achieved.
With greater technology, greater populations, and a greater predictability of demand, the economies of scale increase. Large modern economies can simply produce far more with far less than a subsistence economy. Globalization increases the possible economies of scale even more.
Diseconomies of scale occur sometimes because sometimes the overhead of large groups are greater than the economies of scale achieved by it.
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