I also think that decentralized exchanges are quite convenient, although I agree that Binance and Bittrex are more convenient.

If you just go to the exchange and buy yourself a coin, you can use decentralized exchanges, but if you trade there all day long as traiders do, then you can only use centralized exchanges.
Yes, but this is because decentralized exchanges have small trading volumes. if there are large volumes there, it will be possible to trade more.