In order to perform pre-clearance (or post-clearance), you need to create a sealed area around the stopped trains, which ensures that nobody can pass between any public area and the train without passing through the border checks.
Such a “secure area” can usually only be created adjacent to a side platform, like Amsterdam Centraal’s Platform 15 or Rotterdam Centraal’s Platform 2. In order to do the same at an island platform, this video of Brussels Midi’s Platform 3 gives you an idea of how much platform width you need to physically separate the two tracks which serve the same platform:
https://youtu.be/VJ9M7xNiYvo?si=OQHwooe2NCZVl74b
Toronto Union has and will not have any remotely sufficiently wide platform to physically separate the passengers from both platform tracks. Similarly, it has and will have only one track with a side platform (Track 1) and as every single other track at Union Station it lacks the adjacent apace (horizontally or vertically) to create the facilities where customs and immigration checks could be performed in full segregation from other passengers.
It’s for the same reason why Amsterdam-London Eurostar trains don’t stop at Schipol Airport or Antwerpen Centraal and why the idea of direct trains from Frankfurt and Cologne to London has died, because neither Schipol Airport, Antwerpen Centraal, Köln Hbf nor Frankfurt Hbf has the space to create dedicated and segregated facilities for UK-bound passengers…