The idea with a pre-clearance facility is that there are separate waiting rooms, restrooms, and other amenities so that passengers can enter the facility as they arrive (by bus, taxi, other public transport, etc.) and people using the pre clearance facilities are not subject to long queues to clear customs, so it makes sense, if Amtrak is running a cross border service with 1 stop in Canada and 10 in the USA, to use a pre clearance facility in Canada. Were such a facility to be built at Michigan Central, the sensible thing would be to have Canada Border Services Agency establish pre-clearance there, but that benefits people traveling to Canada on VIA, not people traveling from Canada on Amtrak (and suggests that VIA trains would be extended to Detroit, not Amtrak trains extended to Windsor).
In the case of taking the direct flight from Dulles to Ottawa, that means dealing with customs only in Ottawa, which means walking the ~600 ft through customs from where the taxi drops me at the airport (I fly this route relatively frequently and the longest line I've encountered was 4 people ahead of me) instead of queuing through customs in Dulles with 500+ others as my flight and two or three international wide bodies all land at roughly the same time.