Nexo (and others) use the stablecoins (and other crypto) for liquidity pool and/or for leverage trading. Hence the coins generate profit which is shared with the owners. Plus, when you deposit crypto to Nexo, you don't control it - not your keys, not your crypto.
HBD in Savings does nothing. You have the keys. Have you thought about where does the HBD interest come from?
I wrote down a post which generated interesting discussion. It tries to explain why this interest rate is not beneficient to Hive. It may be worth reading ;) And I would not skip the comments ;)
Nexo has a for profit model for sure and it generates profits for the company and users alike and once coins submitted their wallets there is no control like as you said having keys. True.
I'll read the post and see how interesting it is to churn Hive for HBD interests.
Thanks for the heads up..