When America was first being mapped, the mappers started around Washington and started mapping out the states in the conventional way, putting boundaries down the middle of rivers, around landmarks and suchlike.
After a few months, but a few states were mapped, and the mapping team was told to speed up their work. They decided, therefore, to divide up the remaining states with lines parallel to a longitude and latitude grid.
This is why the Eastern states have wavy borders, and Western states are more geometric and square-like.
Mapping the states took about 6 months.