There are many reasons against this approach:
1. It would mean that you had to be significantly earlier at the origin airport
2. It would kill MCTs to US based flights with current schedules. Add 30 min minimum, every time
3. It would, as noted before, mean significant changes to the airport layout, including lounges etc.
4. At several airports, there are many US bound flights during a short period of time and then none for a longer time. So there would be a huge queue and then nothing for a long time
5. It is generally questionable if foreign authorities should be allowed to work within another country's border
In total, it would be a much better approach to deploy the additional staff needed for these security checks to the US destination airports where they belong.
Regards
Stepfel