This probably depends on your location, but that it very much not most people in the UK. Drinking alcohol almost everyday would be considered normal here. And a lot of socialising is organised around pubs where it is customary to drink alcohol. So for many people, it may have been a long time since they have gone weeks without drinking alcohol. And they may be curious about what it is like.

Of course, not everyone drinks this much (or at all), but perhaps it gives you an idea of the sort of context in which being "sober curious" make sense as a concept.