Not the full story coinbase also custodies assets on behalf of funds and ETFs where people don't own the keys and have no idea what keys are they just know they own some bitcoin in their portfolio.

Same with people who own ETFs that have a piece of it in bitcoin.

As for bitcoin, yes it has the highest self custody rate compared to everything else, while shitcoins like ETH have low self custody rates due to the fees

Also a lot of people don't have the technical knowledge to do staking so they do it via exchanges like coinbase and binance

There's a lot more exposure to CEX"s and people are going to get Quadriga'd and Gox'd