bitcoin is the slowest of all cryptocurrencies and many people use alternate cryptocurrencies (colloquially called altcoins) that transfer to exchanges faster than bitcoin (2-10 minutes compared to 10-60 minutes)
so storing more securely in your own altcoin's wallet and transferring is very fast.
most people aren't selling bitcoin for national currencies when they are using an exchange, they are trading between altcoins and other cryptoassets.
In the 5% of the time you actually need a national currency, you deal with those fiat exchanges on the far edge of the cryptocurrency economy, and deal with their slowness and questionable security for a brief amount of time.
Also, although less true now, exchanges can't be relied on to broadcast transactions to pay for things as well as a software wallet does. So when you aren't speculating and trying to buy something in a time sensitive way (such as using bitpay or shapeshift.io), using an exchange as a personal bank account for online shopping results in a horrible user experience.