Although we are talking about the technology and philosophy of this business here, we are also trying to make money. Bitcoin does not have a single purpose. Transparency of transactions other than anonymity encourages us to use it. 
Decentralized exchanges are a place where your money is safer than centralized exchanges but people who do not know about a cold wallet or decentralized exchanges may prefer to use centralized exchanges as a wallet. There is also the trade and liquidity part of the business. 

Dex's do not promise liquidity. Cex, on the other hand, can respond to your requests thanks to its high volume and liquidity. By using a centralized exchange, you will not be doing anything against Bitcoin principles.