The problem is that we still have to take into account things like population density and urban growth.
Iowa is over 90% farmland, and its capital city alone (Des Moines) only has a population of around several hundred thousand. Its population is more evenly spread out within the state.
On the other hand, the East Coast, which is dominated by massive cities from Boston to New York to Washington D.C., has immense concentrations of people that is far from being as neatly spread out as Iowa’s population. Irregular county shapes are pretty much necessary.
I agree with you, gerrymandering is bad, but just placing a grid over a state and calling each resulting cell a county isn’t the best way to get rid of it.