Because generally speaking people enjoy living life and as a species that means socializing. Actions required to limit the spread of infectious airborne disease

Because generally speaking people enjoy living life and as a species that means socializing. Actions required to limit the spread of infectious airborne diseases are directly counter to that. Fostering global trade and free travel directly increase the spread of certain diseases but overall improve living standards. As a species our behavioral patterns have evolved to prioritize socialization & personal contact as more beneficial than disease risk, despite the impact of even more deadly diseases historically (e.g. black plague in Europe killed 1/10 people for hundreds of years in europe).