Europe has numerous towns - like Dinkelsbühl, Germany - that are preserved in roughly the same state they were in at the time of building. In Brazil, Ouro Preto and Mariana are similarly preserved.
The United States has some areas that bring back memories of colonial towns. Many of them try to recreate the experience of colonial America, like Colonial Williamsburg and parts of Plymouth, but as far as I know there isn’t an example of a preserved colonial town in the United States, let alone one where people live normally and don’t exist to recreate the actual experiences of colonists. Why is this?
So this question isn't answerable because it makes an incorrect assumption. There are many towns that were started in Colonial America and are still relatively preserved today. Many of these towns, like Annapolis MD, date back to the 17th century, and still have the core of their city still preserved today (in Annapolis' case, the old state house and the downtown district are all preserved. There's multiple historic house museums and other public preservation places in the city as well). Now in Annapolis' case, the city greatly expanded during the 19th and 20th centuries, leaving the heart of the old city in tact, but expanding all around it.
As in the case Dinkelsbühl, the core of Annapolis has plenty of its centuries old buildings both perserved and in use, however, if you go into the immediate neighborhoods surrounding the old parts of the city, you'll see modern construction that was build in recent centuries. Now while this gradual change can be seen in Annapolis, which led to the core city being preserved, many other cities created a pattern of destroying old buildings and erecting new ones, as seen in downtown New York.
Unlike Annapolis, which only needed to expand during the 19th century, New York City saw constant waves of immigration through the 19th and 20th centuries. The constant changes led to property owners tearing down old buildings so that they could create ones that met the needs of their time. It's important to realize that to someone living in the early early, say around 1850, didn't necessarily view buildings made 50 years earlier as something worth preserving. As the industrial age caught up, there was a need to create larger, more efficient buildings, leading many places to race old structures and create new, often much taller structures. The process repeated itself in New York for many decades. This is a key difference between cities like Dinkelsbühl and Annapolis versus New York, since the first two cities could keep up with changing demographics simply by expanding, where as cities like New York needed to reshape the 'old' city to keep up with current changes.