I think you are confusing two different actors in the market, investors are interested in the long term and once they select a coin they are not going to be interested in selling their coins even if the market is going down, traders on the other hand are interested in the short term fluctuations in the market and as such when there is an important decrease in the price they need to set a stop loss as they know that being part of the market when it is going down could take them months to recover in the best case scenario.