Number of events often reflects demand. The Brazilians, Argentineans, Chileans, Colombians, Venezuelans all have their own relatively homogeneous calendars, and many of the teams will not cross over. You didn't see Orgullo Paísa and EPM-UNE in the Copa America de Ciclismo or San Luís, nor do you see Clube DataRo and Funvic in the Vuelta a Colombia. It is unfair to count up their number of events as one because the teams will not ride all of them. If more race organisers had the wherewithal to take UCI categorisation, then the US could match the South Americans for UCI points. None of the South American races are above .1, in fact only San Luís is above .2, whereas North America has 2 ProTour events and 2 HC races. You've got bigger events with more points at stake, which offsets the problem of having fewer events.
But this is patently false as well, because there are a huge number of slick and professional teams in South America without UCI categorisation at all. Boyacá Orgullo de América have no UCI status, nor do Pinturas Bler, Lotería de Boyacá, EBSA, Gobernación del Zulia, GW-Shimano, Lotería de Tachíra, OGM, T Banc-Skechers, São José dos Campos or São Lucás Saúde. Those guys are genuine and tough competition for the riders in UCI rated teams, and they take points away (even though they can't actually get the points themselves). And again, the US may have fewer races to get points in, but they have more points available. Rather like South Americans picking up intermediate sprints à la Thor Hushovd, and North Americans being able to pick them up in stage-winning huge bites like Mark Cavendish.