Previously I also used to think like you until a friend of mine started using Coinbase Custody. Coinbase Custody provides you a good amount of services along with advantages. If you are a day trader, handling your own privates key could be sometimes difficult, which is why Coinbase Custody is used.
Yeah really...
So your fried has a 10 million balance on Coinbase and has paid the 100 000 $ for it?
Has also your friend understood that this service is not for traders as in trading on exchange but acquiring large sums of crypto, as those are using their funds daily, this is only for businesses that are storing those coins there?
Be honest, in reality you don't have a clue what this service is about, do you?
Many people who are not technology freak finds it tedious to take care of their own coins. They find too geeky to maintain their own wallet. Thus different companies and institutions out there are taking advantage of such people and providing them those services. People start to trust them with their coin not knowing the risks just to keep themselves away from maintaining their own coins.
It's not only that.
In a large corporation or a multinational business it's pretty hard even from legal point of view to entrust the private keys to somebody.
It's like having cash and storing it in the houses of the CEOs.
So, a service like CC which is more like a bank with a secure vault is the easier way to solve problems.