There's an old saying "Buy when the market is green and sell when the market is red".
It's arguably one of the better strategy as your money won't remain stale. When you see a bullish trend, you buy the first moment you can and when you thin it's turning into a bear, you sell. Short term investors earned a good profit in last 1 month as compared to holders that are holding for last 2 years. Different people have different strategy when it comes into trading but the essence is to increase your profit while limiting your loss.
I think you nail it perfectly about why people doing it. Not just because of FOMO alone but there are short term traders make an investment like this by 'surfing the wave' as I call. If they are too greedy and refusing to sell when there is a sign of bearish, their loss.