Economic growth is defined as the increase in production or output per unit time. Why isn't it instead defined in terms of the increase in national wealth?
Because the word economic in economic growth refers to growth of economic activity. By common definition (see Eurostat)
An economic activity takes place when resources such as capital goods, labour, manufacturing techniques or intermediary products are combined to produce specific goods or services. Thus, an economic activity is characterised by an input of resources, a production process and an output of products (goods or services).
Wealth is an accumulated difference between production and consumption over time, since any production not consumed will eventually become part of wealth (typically measured in terms of net assets).
Thus wealth is not in itself economic activity, as defined above, but rather an accumulation of the output from the economic activity that society did not decided to consume.