Only because thats usually how you end up with it in countries where its illegal. Common Joe doesn't have the skills and equipment to turn it to something non-harmful for consumption. I am amazed how quickly in US the oil vaping industry had developed, and how good the products are compared to old school consumption.

Compared to Netherland where this is cca legal for many years, and you can still only buy plants or cakes (which isn't ideal for quite a few reasons, ie hard to get right dosage first time). In fact, oil vaping cartridges are illegal there (no sense whatsoever, you can buy hashish which can be very potent).

I can guarantee you, tons of people who don't smoke cigarettes vastly prefer consumption with 0 carcinogens from burning the plant.

I haven't read any studies in this. Vapor absorbed by strangers must be extremely low or this would be a bigger issue.