Because it isn't at all. More infectious means more people getting sick, means more people needing health care, means risks of not having enough hospitals beds
Herd immunity comes with the risks of too many people being sick simultaneously, needing oxygen and could mean hospitals beds and ventilators won't be enough to help all sick people when needing it (and eventually dying from a lack of oxygen while waiting for a ventilator to be available). I'm pretty sure Bergamo showed us what the risks are what happens when too many get the infection at the same time.