I've never heard that Bloor-Yonge is the busiest interchange station in the world before, but I wouldn't be overly surprised. Ontario has a habit if building infrastructure in a way that gives us a few massive pieces of infrastructure but poor overall coverage. Compare with Quebec for example. They have more freeways than Ontario and Montreal has more downtown train stations, more freeways, and a bigger subway system than Toronto (not that all of these are bad things). And it's not just Quebec, Alberta has more four lane highways than Ontario thanks to their extensive at grade expressway network.
Ontario has a bit of an "all or nothing" attitude when it comes to infrastructure. We have a barebones subway network but the lines have massive trains. Madrid, for example, is the complete opposite. We have a relatively small 4 lane highway network (by North American standards anyway) and hardly any at grade expressways, 2+1 highways, or super-2s...but at the same time we have the busiest highway in the world. We have very little redundancy in our systems, as the bridge failure in Nipigon showed and as we can see every day at Bloor-Yonge.
I don't know why Ontario is like this, but it's something I notice every time I travel.