I think it would be hard to require, because they may not exist. And it's not a requirement in the sense that you can't come here without them; it's more that if they're requested, people will do their best to bring them. Then, when they arrive, especially with kids, they can be caught up.
So you wouldn't stop anyone coming here, but knowing what they have received allows us here to make sure they get any immunizations they've missed, which, again, protects Canadians, because then they're not going to put other people at risk.