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 ;)