Why aren't there any ancient ruins in North America? South America has plenty of ancient ruins like Machu Picchu, Kuelap, and Tiwanacku. There aren't any major ancient cities in North America.
Edit: I understand there are technically Aztec ruins in Mexico, but I was more talking about ruins in the United States/Canada.
Honest question asking for clarification and not just me being snarky: Do things like Cahokia or the various mounds built by the Hopewell not count? There are also many remaining structures in the Southwestern United States.
If none of these count, could you please rephrase your question to explain why and what you're looking for instead?
Depending on your definition of "ruin" (there are archaeological sites dating back more than 10,000 years in North America) there are certainly very many. Two main regions have traditions of building permanent structures, as already mentioned by others, in the Southwest and eastern U.S. The Southwest has a long tradition of stone and adobe architecture while the Southeast and Midwest have a long tradition of earthen mound construction.
In the Southwest (southern Colorado/Utah, Arizona, New Mexico, and the Mexican states of Sonora and Chihuahua) there are several thousand years worth of ruins of villages and towns. Some of the earliest permanent settlements north of Mexico are in the Southwest.
Just as one example, the site of Cerro Juanaqueña in Sonora is an example of a terraced hill settlement dating back to about 1000BC. You can see a better picture of the site here with the terraces in sharp relief. Note that the entire hillside has been modified by humans into walled terraces, estimated to have required the movement of more than 20,000 tons of earth and stone. Edit: The 20k figure is on the low end and the 40k from the link (from one of the archaeologists who worked at the site) is on the high end. Actual value somewhere in that range.
As already mentioned there is a long tradition of building large earthen structures in the eastern U.S. One of the earliest (and largest) is the site of Poverty Point in Louisiana which dates back to about 1400BC. Again, a massive construction involving the movement of tons of earth and also a world heritage site.
In both these areas, these very early examples are followed by almost 3000 years of building fairly permanent structures which resulted in the very many ruins in the present.