A country that exports goods to the US earns dollars that can be invested in the US. So, one way to look at it is that American consumer spending funds foreign investment in the US. This consumer spending could alternatively fund US exports, but doesn’t if foreigners prefer to buy US investments. (They might buy Apple stock, for example.)

If Americans spent less and invested more, they might own more of these investments themselves, but they also wouldn’t have as much stuff, and foreigners wouldn’t have as much money to invest in the US.