Most of the funds are kept in cold storage for security purposes (since hackers can't touch it) and the remaining funds that that are moving are kept in a hot wallet otherwise It would take forever to process them.
I guess that's the safest ways on how to take care of our funds, and these days as cryptocurrency involved lots of money the hacker is merely aggressive to attack those vulnerable accounts specially in trading sites. With regards to withdrawal issues of exchange to another exchange, you shoud have an authenticator app in order to have much secured access to avoid any entrusion from hackers. Having tight security on your funds could definitely give you more safer funds transfer.