If you need to cover your short and there are no shares being sold you effectively must keep offering higher and higher prices until someone will sell you a share as you are legally obligated to buy a share[1]. To end the unlimited downside (of raising the price you'll pay by more and more) you need to have enough sell orders on the books to cover your short or have raised the price enough for someone to sell. During the time you are trying to buy your loss is increasing (as you raise the price to get a seller) and is unbounded.

If you have a stock you have to sell and there are no offers to buy on the books you can't offer to sell then keep raising the price, instead you would need to keep lowering the price. The price is either limited by what's on the books, or you need to wait an indeterminate amount of time for some one to buy at the price you are selling. During this time you are trying to sell your profit is decreasing (as you lower the price to get a buyer) and bounded by $0 (for a limited liability company).

So while it's true that if you hold a stock indefinitely the value you could get is unlimited with a short the value you could lose could be unlimited over a much shorter time frame.

[1] Whether you actually need to cover your shorts may be a matter of some debate if you look at the wild rumors around GME