To a large extent they normally are "stored" in the air. A typical narrowbody aircraft in the US clocks 9-10 block hours a day (a little less in the air) and widebodies even more. So somewhere around 40% of all commercial passenger aircraft are in the air at any given time (higher during the day and lower at night, of course).

Keeping a plane in the air requires extremely expensive amounts of fuel. These planes would otherwise be active on routes, at airport gates, on the ramp temporarily, in maintenance, or stored at a base owned or partnered with by the airline (in the worst case). A big part of running an airline is making sure you are using your fleet to make money since planes are expensive to own, maintain, insure, and fly. These airlines may be switching over to freight for supply runs but don't have their own storage capacity for the planes that would normally be flying now-closed passenger routes and can't be repurposed for freight.