Some investors are generally interested in buying more BTC at DIP prices and tend to sell during bullish times. But holding is more important because it will happen again and again when ATH is observed in the market with gap of few months. A bullish period is characterized by overall market optimism and expectations of higher performance for the future. Even offers higher returns than bearish ones. A bull market usually reflects the utmost confidence of investors that a person can align their investments in this positive trend. Increasing market value makes investors optimistic about the positive loop. These positive market conditions may prevent investors from being psychologically tempted to sell BTC. It can even discourage selling during market downturns. Many investment firms can build potential wealth by investing more during periods of BTC price appreciation.
I don't think there's anything wrong with buying low and then selling high, that's literally where the money's at in most investment industries, and imagine if everyone just held their bitcoins for dear life, there will be a bubble that would literally implode upon the first dollar sold on this network. It's good that people are holding and I can't and won't disregard how important it is but so is buying and selling, these things are what keeps the wheels turning in the bitcoin network, if even one of these things go away we're fucked.
Even buying when the coin's at a high price point is just as important, it keeps the interest coming for the peeps on the network, makes them think that there is still a chance to make bank even if the price point is this high already (which they are).