I think the reason people still keep their coins on an exchange is that they don't want to lose that small part of their money which they should pay for withdrawing. This approach is not a good one though. You should easily accept losing a small fraction of your money in order to not lose much more.
That is really a foolish thing if really someone thinks that, being that much miser would probably cost you, for just paying 0.1% or something on withdraw fees you put all 100% of your money under risk? Is that wise at all?
It's an illogical way of thinking. I've bought $100 worth of Monero, but I couldn't buy it all at once so I had to buy 5 times to reach the sum. After 5 transactions, I have lost 1 Monero which means $10. This means I'm left with $90.. It's a misery but I still prefer losing $10 rather than risking $100.