Food production is a huge one. We don't want highly optimized farming where only the most efficient growers feed everyone else because that has the risk of global famine if something fails there. The more a system is optimized, the closer to failure it is. Same goes for all other kinds of production but food is really important compared to, say, CPUs or cars.
Fine .. say your country has a several years of drought and bad harvest. What happens then ? Do you trade then ?
Or .. lets say due to weather, your farmers can not grow enough oranges or some fruit which drives up local prices. Should only the richest people in your country get to eat fruits ?
Or you discover lithium deposits that your national industry can not use . Should you let that just sit there knowing it could make your province prosperous if traded.
You took a far too extreme interpretation that ended up backwards. What normal countries do is trade anyway but with tariffs and subsidies so that in normal circumstances, local food is competitive to keep farmers operating but if there's a local production problem, imported goods become more competitive. That buffers the population from extremes of price/availability.
