That's great if you feel you can time the market to your advantage. The truth is, though, that most people can't time the market and/or don't have the time to put in the research and learn how to do so confidently. For many people, holding long-term is a better strategy than selling and trying to buy back in lower to increase their stacks. Many people have tried to do this before and missed huge gains when the market unexpectedly turned around -- it's not always worth the risk.
Great if you can do it and get it right more than wrong. Not everyone can.
Plus, there are tax considerations. Every crypto-to-crypto trade is a taxable event in the United States. Many people have decided it's a better idea to hold for over a year rather than trade -- long-term capital gains vs. short term, easy-peasy reporting come tax time vs. a great big fucking mess that will make your CPA want to murder you.
And some people are simply less stressed holding vs. trading. Less stress = longer, happier life.
There are plenty of smart, logical reasons to hold, assuming you believe the market will rebound at some point and it isn't all downhill from here.