Given that Treasury bills are theoretically assumed to be risk-free investments (not going into whether they really are or not), is there any reason anyone who is planning on keeping the bonds until maturity doesn't just leverage these as much as possible? The way I understand it the lack of risk of the bonds becoming worthless as well as the fact that you'd always get your initial investment back upon maturation (as long as you're not buying them for more than their underlying value) means that buying these with leverage would in theory have no downsides.
Now, this to me sounds like a "too good to be true" scenario, so I was wondering which risks and other issues I'm missing here, at least aside from the fact that the option of a country defaulting does always still exist.
As an aside, this is not something I'm actually planning on doing right now, since the dissonance between 'risk-free' and 'leverage' sets off all sorts of alarm bells in my mind.