we're bigger than they are.
it would serve no purpose and give the russians an excuse to invade canada at the same time. we don't want a canadian molotov-ribbentrop.
there's a section in their constitution about this. it's not crazy; they always intended to, but they never did.
what if they just invaded ontario, quebec, alberta and bc and said "fuck the rest, let 'em sleep with the meese"?
well, they'd better start patrolling the north, but i don't think canadians would tolerate it, and they'd have to deal with canadian militias forever.
it's a better idea to just try to be nice to us, which is why every president for decades has done that.