The question popped up when a friend complained about "McMansions" and praised the Historic neighborhood I live in (Irvington, Indianapolis, IN). But as I drive through my neighborhood, the architecture varies only slightly between houses, about as much as current tract housing, in which owners can pick a floorplan, materials, and additional variances. I did some research and found Levittown, NY is the most famous example of this, but my home was built in 1934, years before Levittown was built. Although I can't find another house exactly like mine, there are others very similar. Also, there are other houses that are identical from street to street. So was the practice of building numerous houses from few plans normal? Was this just an instance of wanting your house to "fit in" when it was built? Or architects giving discounts to reuse old plans? Some combination? And most interesting to me (though perhaps outside the purview of this subreddit), why do people deride current tract housing while ignoring that this has been around for some time?
Subdividers who also built homes for sale were already common in 19th century American cities. Samuel Eberly Gross is the best-known example in the Chicago area because some of his promotional material survives, but many others must have built a block or two of new houses at a time. More commonly, though, subdividers wanted to sell the houselots quickly, either to families who’d hire a builder later, when they could save the money to build; or to builders who might purchase three to a dozen lots at a time and build on spec. These were not heavily capitalized enterprises, however, and generally they needed to sell houses almost as fast as they could build them. Subdivision and building companies got much larger during the boom of the 1920s, almost to the scale we know today—but then came the Depression, leaving the industry reeling. The Chicago area, for example, was left with thousands of vacant houselots on hundreds of empty streets.
During the 1930s, modern industrial mass-production trends began to intrigue builders and those concerned with housing issues, and the enormous housing needs of wartime industry (and whole new towns like Oak Ridge) offered the opportunity to try new techniques. Greg Hise’s book Magnetic Los Angeles: Planning the Twentieth-Century Metropolis describes how enormous new subdivisions were mass-produced using new techniques, and how those techniques came to dominate homebuilding after World War II.
As you note, there’s often very little difference in variety between the 19th century rowhouses of Philadelphia or Baltimore, the bungalows of 1910s-1920s Chicago, the “little houses made of ticky-tacky” of postwar California or the Levittowns, and the new subdivisions of the 1990s. It’s just that over decades, owner alterations and mature landscaping soften the starkness of rows of new, nearly identical, houses.