For example, arriving on an international flight to YYZ (but continuing on a domestic connection), one clears customs in a clearly sanitized area (where the general public who has not cleared security has no access to), and then one has to exit landside and re-enter security. Why can't the airport allow pax to clear customs and not leave the airside zone, to avoid re-doing the security theatre dance?

Are there any Canadian airports that allow what I've described above?