Why don't we open the border with Canada?

OK, there are lots of reasons, some legitimate, some simply disgusting and hateful, that we maintain some semblance of security at the border with Mexico, but basically none of those apply to the border with Canada. It’s a modern industrialized democracy with a strong economy and less crime than the U.S. has. If Europe can make agreements that enable open borders, then considering that even the problems Europe is having with that aren’t likely to exist between the U.S. and Canada, why can’t we come up with agreements regarding how each country handles customs and immigration with other nations so that we can open the border with Canada? Maybe it’s a drop in the bucket of the Federal budget, but surely we could save a lot of money by not maintaining border checkpoints, and it’s bound to be good for the economy. Maybe it’s Canada that doesn’t want us coming in with our American criminal tendencies?