I think that is because the fee of transaction that makes people thinks that if they putting their crypto with a third party will be safe and they don't have to bother sending their crypto. Not to mention about the transaction fee that can increase following the busy of the network.
Besides that, if their crypto is on the exchanges, they can directly sell their coin in any high price they want. They don't have to wait until the transaction finish which can be longer if the network is crowd. They should realize that there are no 100% safe in the internet even if third party can guarantee that all data in their site is safe.
For people that sell off are not long term investors, if we actually are taking about those that should take off their funds off exchanges I think the advice or rather warning is usually directed to those long term holders with huge amount of funds. For short term holders or even day traders no problem since it will cost them extra charges in fees.
The transaction fees I think it is only when you intend to move your funds to exchange and sell that you meant but sending between two wallet addresses the transaction fee of using a wallet is always cheaper than sending from exchanges, as you have the ability to actually set your fees yourself if you use the right wallet.
And another reason is they can grow their money, holding in non custodial wallet will not make them able to grow their money.
People who invest in cryptocurrency are looking for profit over anything, it's fine for them to risk more if they can earn more rather than playing safe. It's very rare to see someone hold their coins in their wallet, if you have real life friends who invest in cryptocurrency, you can check where they hold their coins.
Staking is something I don’t actually think a large holder will even consider except all this altcoin lunchpools by exchange to get newly listed coins for free but for the APR the risk is definitely not worth. One thing is many of these holders of coins on exchanges simply do it just because they don’t want to be their own security and also they think that when hacked this exchanges could offer return over their personal wallet been hacked to me it is simply still a gullible thinking