Why most of them still use a custodial wallets because their don't want to go through the stress of developing their own wallet that can work with their payment gateway that is why they are relying on exchanges such as counbase.
Possible reasons:
- They don't know how does bitcoin work nor why was it created. (And they don't care to find out)
- They just want to have a quick profit.
- They feel more comfortable if another entity holds their money.
- They're lured from the fancy buttons and stylish look of the exchanges' website. (Which, definitely, includes the first one)
- They want to transact only with other people within the exchange. (Which reduces their fees)