Detroit becoming a hub of automotive manufacturing actually had a lot to do with the fact that many of the founders of those successful companies were from or had moved to Michigan when they established their companies.

Henry Ford was from Greenfield Township, which was later absorbed by the city of Detroit. He built his first automobile in Greenfield and went on to establish Ford Motor Company, his second attempt at a automotive manufacturing company (the first which he left in 1902 later became Cadillac), in the Mack Avenue Plant in Detroit in 1903.

General Motors was founded by William C. Durant, who had also founded Durant-Dort Carriage Company in Flint, Michigan in 1886. By 1900, this was the largest carriage manufacturing company in America, producing over 100,000 carriages a year. GM was actually originally a holding company through which Durant rapidly purchased a number of automotive manufacturing companies, including Cadillac, Oldsmobile and Oakland (later known as Pontiac).

Ransom E. Olds, who founded Oldsmobile and is credited with the creation of the modern assembly line concept, was born in Ohio but later moved to and settled in Lansing, Michigan, where he later established Oldsmobile.

Buick Motor Company was also founded in Detroit, but in 1903 was bought by James H. Whiting, owner of Flint Wagon Works, who moved the company to Flint.

There is certainly an argument to be made that the presence in Michigan of some of largest carriage/wagon manufacturers in America had a significant influence on the development of the automotive manufacturing industry. After all, carriages and automobiles shared a large number of components, such as springs and axles amongst others. Though it is from Indiana rather than Detroit, Studebaker is another wagon manufacturer that later transitioned to automobiles.

This is one of the most complex questions in urban economics. The trouble is not that there aren't any quantifiable factors, but that there are too many. Factors like transportation, access to skilled workforces, access to capital and the like all seem to be important to the emergence of an economic cluster like Detroit or Silicon Valley.

But they are clearly not enough as places with those factors don't all become clusters. Chicago, Indianapolis and a few other Midwestern cities (and even Springfield, Mass) had roles in the early automotive industry.

Perhaps the most important factor isn't geographical or economic, but the human appetite for risk: Henry Ford and William C Durant took massive risks with their own and other people's money (Durant eventually went bankrupt and near the end of his life ran a bowling alley and sold hamburgers). Some founders test-drove the cars they made.