why short call is a liability? like, management, liability, example, written

Wendy01

I would like to know why short a call is a liability.
In the derivative application for risk management study session the concept is written on the Schweser notes.

When short a call, get a premium for example $ 50.
Call seller has now U$ 20, then why it is a liability?

If the call in the money, say + 100 at expiration then the payoff to the seller is - 100 & profit -80
If the call is out of money then the payoff to seller is 0 & profit +20

with this trait, how a premium on short call seens as liability? when in the money, the premium 20 protected the loss for amount of 20 (loss 100 to 80)