Most of those airlines are not American. Preclearance would never work in big airports anyway due to passenger volumes, it would be even worse than NYC.
YYZ CBP handles about 8M pax a year. Not many airports in Europe have 8M pax to the US.
Airports just need to be designed to handle it. The large airports in Canada make Dublin's CBP look pathetic, with large post-clearance areas with shops, bars, waiting areas etc.