The only trading strategy which have potential to work in an unpredictable market is long term trading, which involved the trader holding for a long time period, up to few years... That is why patience is needed. On the other hand, for daily traders there aren't any assurances they are going to make guaranteed profit from this activity in a daily basis, therefore to rely a living from trading is too risky due to market's conditions.
Holding for few years is just what we generally regard as investment since trading is seen from the angle of a couple of days or months before the selling. I think it is necessary to make that distinction for the purpose of the discussion because even short term investment is not too different from position trading which involved holding for several months and sometimes years.
And doesn't matter how professional and skilled the individual is on trading matters, he still can't forecast the next movement of crypto market accurately. These people making a decent living from trading you mentioned must have another sources of income to ensure their finances continue doing well in case trading fail along the way.
You are right because it is absolutely difficult to predict the movement of the market with accuracy. When you consider the stress involved and the risk you will be taking, you will realise that it is not just necessary because the stress is not worth it when there are better alternatives such as simply buying and holding.
If two investors start their Bitcoin career with the same capital, one chose to start with trading while the other chose to buy and hold, there are high chances that the holder will become more successful than the trader. This is why .any people feel it is more comfortable and reliable to invest and hold than to trade.