Why don't airlines place some physical separator between passengers in the planes to prevent the spread of COVID-19?
Cost. Those would be expensive, and the business is bad already. Now, you might think that just any mesh, fabric or membrane would do, but but to be effective against the sperad of viruses, nope.
Discomfort. Imagine how it would feel to be bagged up like that for hours. Again, despite the membrane being a &quot;breathing&quot; sort, it would still get rather hot and huimid inside that contraption very quickly.
Restrictiveness. Imagine an evacuation situation with people in those. Very dangerous, not gonna work.

Masks are very effective, cheap, and people are quite willing to use them (excluding some mentally challenged cases). The ventilation in planes is also rather safe when considering the spreading of viruses (see this ASE Q&amp;A), especially when combined with the use of masks. Planes usually have very good filtering of recycled air, and much of the air is not even recycled.