<< how come people say that the American dollar is worth more than the Canadian dollar? >>
Due to their ignorance of how exchange rates work. The American dollar IS worth more than the Canadian dollar, but that doesn't necessarily mean you get more purchasing power. 'Money' is worth exactly what it will buy, no less and no more.
Some people will see that Product A costs $15.90 in Canada but 'only' $10.00 in the U.S. and think, 'Wow, things 'cost' more in Canada."
Others think that they're getting 'more' money when you exchange to Canadian dollars, except $10.00 US and $15.90 Canadian buy roughly the same thing (on average according to the exchange rate today).
There is some exchange fluctuation that doesn't immediately affect the price of an item in either country, but unless you're dropping some serious short-term money, you're only saving a couple bucks. Tariffs, taxes, and duties can screw everything up.
In Afghanistan, a wheel-barrel full of Afghan money might buy you a loaf of bread. Nobody says, "Wow, I've got one hundred thousand Afghanis ($21 US dollars), I'm rich!"