Why can Americans get into Canada with just a passport, no visa?
Countries sign mutual agreement for less paper work for their citizens to cross each other borders. Usually they share immigration database and records of each others, so national IDs or driver license should be enough. Its not exclusive only between Canada and USA. Nepal and India have similar agreement. Central American countries like Guatemala, Nicaragua, Honduras and El Salvador have similar agreement as well. 10 ASEAN countries citizens can visit other ASEAN countries visa free for up to 30 days.