It is the infection per population that is lower for seasonal influenza, because the population is far from naive; influenza circulates, so the majority already have immunity, and thus the population as a whole has resistance.
SARS-CoV-2 is brand new; without restriction is will cut a swathe through the population. It will be years before it is a circulating disease like seasonal influenza. In fact, it may never become such; we know so little about it, recovery may not even confer immunity. We do not yet know. The restrictions buy us time.