According to this answer (and many others I've read), there are US customs officers on Canadian soil.
What about the reciprocal (Canadian, Mexican, Japanese, etc)?
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There are no preclearance facilities in the US for other counties. It would be possible to have them with appropriate facilities and negotiations over legal and logistical arrangements. For instance, the US-Canada preclearance agreement is reciprocal; it contemplates Canadian preclearance could be sited at 13+ US airports, though that part of the agreement has not been implemented.
There would be hurdles to doing so, including designating airport space for the facility, arranging the airport to have a secure area for preclearance flights to arrive/depart so that precleared passengers are segregated, costs, etc... One difference is georgraphy: the US can cover most flights from Canada to the US with nine preclearance locations in Canadian airports, while Air Canada/Air Canada Express/Air Canada Rouge alone has flights from dozens of US airports. Unless air routes were massively reconfigured in favor of connecting flights, Canada would need a prohibitively large number of preclearance facilities to cover even a majority of inbound flights from the US, and many of those would only be open for 1-2 flights a day, some just seasonally.
Some embassies and consulates in the US may have a customs attaché present, which I suppose is a type of customs agent on US soil. They would be more involved with providing advice on imports, trade facilitation, and security, but not the actual inspection of inbound travelers.
The arrangement of having foreign immigration and customs controls before departure is sometimes known as juxtaposed controls, and they are used in a few other parts of the world, including cross-English Channel routes in the UK, France, and Belgium (+the Netherlands in the future), Singapore, Malaysia, and (avoiding a political debate, we'll just say for some definitions of "foreign") Hong Kong and China.