DEX not always convenient, for example on IDEX, the day before yesterday I had a transaction for more than an hour and I lost more than 10% of the cost, because I did not manage to sell at a high price. on the classical exchange this does not happen!
You can tell about this advantage to those who traded on Mt. Gox
The ETH and erc20 token transaction fees are nowadays horribly high.
It is impossible to trade without be eaten by the fees...