What Eric said is correct - the main issue is that solder paste will only stick to a metal that it can alloy with. If you tried to reflow the solder paste on a bare substrate, it would just ball up.
As it turns out, making sure that the ink will print nicely is a large part of what makes conductive ink formulation challenging. The ink needs to have a high metal content to get good conductivity, but also must include components like rheological additives to make sure you can print properly, and binders to help it stick to the surface of the board. It's a careful balancing act that needs just the right mix of everything to work out!