RAM is used as short-term memory because it is

RAM serves as short term memory because it is volatile, losing data when power stops.

Being expensive affects cost, not why it is only short term.

Small capacity limits how much it holds, not its temporary nature.

Being programmable does not explain why its contents vanish.

Volatility means the stored data survives only while power is supplied.

Permanent storage instead uses non volatile media like ROM or disks.