I think the main reason is we just want to earn money . But I think it's not the reason for everyone

I'm only using Electrum offline wallet to store my Bitcoin cause it's free, I don't have to pay anything to use Electrum and I still can control my private key. But if you want to keep your Bitcoin safer I think you should use coldstorage wallet like Trezor, but the price of this kind of wallet is a little bit high at the moment.