By which measure were they less advanced? Tenochtitlan had a population of north of 200k when the Spanish arrived - bigger than most European cities at that time, bar a couple. When you read the chronicles of the conquistadores you realise how advanced they were in many ways compared with Europeans.
Th Maya were contemporary to and very similar to Greece in many ways - definitely more advanced in some aspects of mathematics and astronomy, and had an extremely complex architecture.
The gap wasn’t so big, and in some cases American cities were even more advanced - probably the complex sanitation system of most mesoamerican cities contributed to the biggest asymmetry of all - European cities were a Petri dish of filth and disease.