Can you name a more convenient way of buying and selling Bitcoin aside from buying it from Centralized exchanges?
I doubt you can't say because there's no other way that is consider convenient is more convenient than buying it on Centralized exchanges. I mean yes Bitcoin has been created so that people can transact without any middleman on it, but nowadays, there are many people who are scamming these innocent holders of Bitcoin, thus they find a way to buy more Bitcoin in a safer way, and the way that they see is to buy thru these centralized exchanges.
Why do investors traders retain their Bitcoin there? It isn't advisable to retain, and store any coins in exchanges especially in the centralized ones, but for some investors traders, it's a way for them to sell their coins way faster if they needed it. I mean if they store it in a hardware wallet, then suddenly the price of Bitcoin spiked up and they choose to sell it, they need to wait a few minutes because of the transaction, and in that span, price of Bitcoin might go low already, so I guess that's the reason why they are storing it there.
I think convenient is a subjective matter, I don't think it's not convenient to buy Bitcoin from decentralized exchange.
You're need to create your account by submitting your email and password, then confirm the code which send into your email.
In centralized exchange, you need to verify your KYC and they will ask you to wait for maximum 7 days, they can reject it and ask you to submit more clear version, let's say you've successfully verify your account, in the next month or year, they can demand more pictures, if you can't do it, you can't use the exchange.
In decentralized exchange, you don't need to verify anything and you can link your bank account or other payment, then you can start to trade!