> while hydrogen _IS_ light enough for an airplane

If you do the watt-hours per kilogram calculation the very best lithium ion batteries are something like 255Wh/kg right now.

Hydrogen fuel cell systems for small to medium sized UAS are somewhere in the 700-800Wh/kg range right now, including the weight of the (rather heavy, seriously engineered) ultra high pressure carbon fiber wrapped tanks needed for it, piping, and fuel cell.

Something like a tank of Jet-A fuel is >4000Wh/kg.

Hydrogen itself is light, the tankage systems needed to reliably contain a large volume of it on an aircraft are not.