Miners have to afford to lose temporary because they can not have profit from mining in both bear market and bull market. It is easily to have profit from mining in bull market but in bear market, it is different.

In bear market, they can have very slim profit and sometimes, they even have loss. However, if they can keep mining and hold their mining rewards and wait for a next bull run, they will get very good return at the end. It would be more similar to long term investment for miners. If miners are short term miners, they will have more risk to get loss.