Because they want total control of there funds that is why they prefer the self custodian to wallets. Nexo and its likes act like traditional banks, they hold on to your funds and loan them out to borrowers with the promise of a percentage which is not worth the risk of leaving funds there. Most of these entities end up running into bankruptcy and that is the end for you funds and the so called percentage. So my advice will be like the rest, just keep your funds on self custodian wallets. The volatility of bitcoin it self is a better percentage if you hold for long and buy at the right time/price.

But if you’re to earn more aside holding, then it would be preferable to learn about bitcoin trading. Trading is risky but at least you can have custody of your funds all the time.