First of all, "did" is past tense. How long past? For example, UA used to allow transfer of their miles to Diners Club (and then onwards to a number of hotel chains or other airlines), but that ended a few years ago. I could tell you I "did" it once, but that doesn't mean you or I can "do" it any more.

Second, at the very least, Delta offers some way (no details available for the ineligible on their website) to get hotels (could be certificates rather than points?) for miles, but only to their elite (Platinum Medallion) members. If you're nowhere close to being one (because you don't fly a ton each year), you don't qualify. And since I'm not one either, I can't say whether this is a general deal (and if so, whether good for a variety of hotels or only super expensive resort ones) or only for a few specific hotels, or even whether it's for one-off reservations or only for buying timeshares!