You can - buy T-Bills. You can even buy them directly from the US treasury at https://www.treasurydirect.gov/indiv/products/prod_tbills_gl...

This is pretty much exactly what the banks are doing, albeit at a larger scale. They buy T-Bills, or other equivalents such as Gilts (UK government backed bonds) as they're as secure as investments get - with reduced returns as the compensation for the increased security of the bonds being backed by one of the worlds major economies.

For more information - https://en.m.wikipedia.org/wiki/Government_bond and https://en.m.wikipedia.org/wiki/Bond_(finance)