US preclearance exists in certain airports in Canada, Ireland, Bermuda, the Bahamas, Aruba, and the UAE.
The US-Canada agreement is the oldest one, largely due to the amount of cross-border air travel, and Canada has the right under the agreement to operate preclearance stations for the Canadian border inside US airports, but has never chosen to do so.
I've learned that, when coming to the US from Europe, if I can't get a direct flight, I prefer my change to be in Canada. This way, I can pass immigration on the Canadian airport, while I'm idling between flights anyway. Also, the one time I did it, there was almost no queue (in Toronto).
When I had a connection in Canada, my incoming flight was delayed, the lines were long and I was at risk of missing my connection, so I made the opposite conclusion.
The US-Canada agreement is the oldest one, largely due to the amount of cross-border air travel, and Canada has the right under the agreement to operate preclearance stations for the Canadian border inside US airports, but has never chosen to do so.