Why is Toronto, Oshawa, Hamilton, and Niagara Falls are located on some of the best agricultural land?

The cities were established along lines of trade/ transportation/ communications, as most cities are. It happens that southern Ontario is more fertile that a lot of Ontario because northern Ontario, for example, rests on the Precambrian Shield. On the Shield “what ain’t water is rock” with generally poor conditions for farming. And the climate in southern Ontario is more conducive to agriculture.