Here are the reasons why;
1. It is quite stressful - old traders will soon give up due to mental health problems
2. It is not enjoyable anymore - nobody will stay long in a job that they no longer find themselves comfortable with especially if you are not earning well.
Most of the time we feel this during bear season.
3. Trust issue due to Scam growth - traders never find it safe to use exchanges, even big exchanges.
I have tried trading as a full-time job and I failed. It stressed me so much that I swear it's better to do manual labour than to watch the price of the coin going up and down 24/7 and then be stressed out about catching good moments to profit. It made me anxious and nervous, I completely changed my sleeping routine because I was always trying to keep awake until I could to control my long and short positions. Maybe bots will make trading more comfortable but I can't depend on them with my capital, I prefer to control everything myself.
You also mentioned it correctly. So many exchanges go scam, get hacked and have financial problems that it's not safe to keep your coins on exchange. It's always a wise decision to deposit on exchange, trade and then withdraw immediately.