The commercial relationship between these countries through the NAFTA and now the USMCA, allow businesses to buy directly from manufacturers in these countries without worrying about tariffs.
Given the proximity and the thousands of kilometers of borders they share, make shipping and transportation of merchandise very effective. This facilitates a very fast reaction time to customers, allowing the business to have the product directly from the manufacturer in a matter of days without having to resort to air freight or onerous sums for these services.
For instance, any product manufactured in Mexico could be in New York in 4 days. This gives your business a great competitive advantage against manufacturing in other countries in which the shipping time would be of 30+ days.