Because they are often fair weather friends with no particular loyalty or allegiance to the USA or our values, and will flee for their home country as soon as convenience dictates.
Just like with other aspects of the labor market (agriculture, hospitality) where illegal immigration undercuts Americans, VCs are doing labor market arbitrage and are importing cheaper "founders" who essentially become visa hostages. It doesn't benefit anyone except the silicon valley elite, who also run this site, incidentally.