Explain to a noob: the purpose of "never sell"?

I get the generic idea that long-term gains are better (e.g. 10-20 years versus 1-5 years)

But I sometimes read posts on here and elsewhere and legitimately get the feeling that some people quite literally do not plan on selling any portion of it, ever.

Is there a psychology behind that? I am doing Crypto literally just to attempt to get about £500k. If I can get that, I will not need more. So I guess I am just curious of the mentality of those who currently - at the age of idk 30 or whatever - have a million or more - and refuse to sell.

This is just a curious chat, so no need to melt at me about it for those prone to such things haha :)