We should. But it takes hours to cycle all the air in a room through one. I haven't heard of any covid studies, but don't think it replaces a mask.

>We should. But it takes hours to cycle all the air in a room through one

hours? A random hepa filter I searched up costs $215, has an airflow rate of 246CFM , and is rated for coverage of 361 sq. ft. Assuming a room with that area and a ceiling height of 9ft, that works out to 3249 cubic feet. Dividing that by the airflow gets you 13.2 minutes, which is the time it takes to cycle the same amount of air as the room's volume.

You did the wrong math: you didn't account for air that gets sucked through the filter more than once. So that random filter is generally assumed to be about an hour to filter all the air in that room once. And that is assuming there is reasonable air circulation over the whole room - often what happens is the filter sits in a corner and filters that corner well and doesn't filter the rest of the room at all.

More filters in the room, and different sized filters both exist.

I would imagine money primarily. On top of that, the larger the room gets the more costly and ineffective this becomes.

Who's going to change the filters?