Why is it not recommended to short sell in a bull market?

In a bull market, many cryptocurrencies are continuously reaching new historical highs, with unlimited upward potential and limited downward pullback potential. Take Bitcoin as an example: even if it falls from 100,000 to 20,000, the decline is only 80%. However, no one can say how high it can rise in this bull market; it could reach 150,000, 300,000, or even higher. Those who have shorted from 50,000 until now do not know how many times they have been liquidated. In a bull market, one should go long every time there is a pullback, following the trend.