I read that heavy cannabis users regain their full capacity just after few weeks. So probably if there's any damage it's nowhere near as extensive as in the case of alcohol.

My guess is because they're not really an alternative because they don't produce the same numbing effects many people seek in ethanol consumption. Also, they are still illegal, stigmatized, and difficult to find in many places.

Tons of studies show that weed effects emotional processing, increases risk of of sudden psychosis, and yes, even gray matter loss. [1] This idea that weed is safe due in part to its associations with "Nature" is really a social fabrication. People just fought for another thing they wanted to imbibe that wrecks us in another, not necessarily equivalent, way.

Personal experience is it's one of the few recreational drugs I can't handle as I experience extreme paranoia, rarely terrifying out of body experiences, and always an inability to speak, which is extremely uncomfortable as I deeply value my ability to communicate.

[1] https://www.nature.com/articles/npp201467