This is a bit of a gravedig, but you ask cool Roman questions and I was sifting through them.

You're the looking at it in the wrong direction! That's why the point seems arbitrary. The state was mostly settled from the south up as people used the Ohio River to move into the Northwest Territory. Notable exception of settlers coming in from the Lakes and Detroit to settle around Fort Wayne

The western border of Indiana is a straight line drawn north from Vincennes, beginning at the final crossing of that line and the Wabash.

Originally, the western border was a straight line north from the confluence of the Wabash and the Ohio, just like its eastern border drawn from the confluence of the Miami and the Ohio. However, these matters are settled by Congress and the original proposed borders were considered too generous. As Vincennes was the territorial capitol and situated on the Wabash, they used it as the reference, giving Indiana all of the land inside the Wabash and north from Vincennes.

This sort of cooperation between territorial and federal government is also why Indiana's northern border is not flush with Ohio's. As the Indiana Territory approached statehood, the territorial government petitioned Congress for an extension of about 20 miles north to allow the aspiring state greater shoreline/port access to Lake Michigan.

Source: I work at the Indiana Historical Society and was involved in our 1816 Bicentennial exhibit.