Hi there!
So this is fun easy answer, no one really knows! However, there is a popular theory. For simplicity sake.
Many Urban Development Historians say this was for easy navigation throughout cities. Also in the United States, many cities grew very fast, so it was simple just to give them numerical names rather than trying to create new names.
For example, Philadelphia
The "Old City" portions, designed by William Penn, (which Pennsylvania is named after) designed the streets, and named them after plants and animals. However, when rapid urbanization began, names were made simple, for example "Broad Street". It simply is broad street that runs through the city!
Here is an article which explains it better, and uses a study!