That is the very thing we should avoid, to put our funds or assets in exchange and just leave it there, especially if we don't know the capacity of that exchange, or how credible they are, so for better safety, I agree with you we should withdraw our funds immediately after trading or we don't want to trade anymore.
There are many cases from my friend wherein they are holding their funds to a certain exchange and after a month, it's gone and they can't recover anymore.
Well.. I am saying this out of my own experience. I have lost my coins after the exchanges got hacked. I used to store my Litecoins in Wex.nz, since I didn't had the desktop wallet installed in my laptop for LTC. And when Wex.nz went down, those coins vanished. Now there is no way to recover them, and I lost around $500 worth of coins as a result of that hack.
And when Cryptopia went down, I lost Cryptopia Loss Marker(CLM) tokens worth around $50. Fortunately, I had withdrawn my BTC from that exchange, when they had enabled them for a brief duration. Else I would have lost another $1,500 worth of coins. But many of the users failed to make use of this window and they lost whatever they stored in Cryptopia.