60% less effective doesn't mean that they are not effective. They remain effective, just considerably less so.
Meaning that if given the choice between "no mask" and "loose bandana", everyone should be choosing "loose bandana" as it affords more protection than nothing at all. It doesn't "block nothing" as you asserted.
Again, I'm going with the conclusion of medical experts and multiple studies. Face coverings, even loose fitting bandana's, remain better than going around uncovered and afford protections for you and those around you, even if they're a less effective method than tighter fitting alternatives.
The problem in the US isn't that people are wearing bandanas, it's that they're not wearing masks at all.