In the past, I didn't really understand this, but I made it simple by using the same matrix as they do in banks and digital cash applications.
You see money as a number in those industries. Either in your cellphone or in your computer. But it remains a number until you cash it out. Real cash. That real money that is in circulation unlike what you see in your digital cash application.
It's only safe and as your own money or asset if you cash it out or if you transfer it to buy something or transfer it to someone for something in exchange and the receiver use it for his purposes.
Without using that digital number in your bank account, you can lose it anytime.
It can be the same way in Bitcoin.
You put in exchange or a trading website it becomes a number only that is visible to your eye. But once you withdraw it and put it in your hard wallet, that becomes real Bitcoin. It's not just a number anymore, it's Bitcoin.
I think for those who are having trouble understanding this, we must simplify it by using what we experience now in the financial industry.
It's same with Bitcoin, exactly if people store bitcoin in accounts on centralized exchanges.
They will have risk of losing their bitcoins anytime because without private keys, they can not send coins.
While relying on centralized exchanges is risky, because exchanges are even unsure about their future, not yet about money of customers stored there.
Reminder: do not keep your money in online accounts.