And when you're not pulling on the J, where does the smoke go? Yep, into the air. Bye, bye THC.
Which is precisely why you pass it, so that when you arn't hitting it somebody else can.
Whatever is lost in the air as smoke is probably made up for by not having to repeatedly re-light and expose the herb to more heat than necessary.
As for resin, you can't scrape it all, THC degrades; resin is basically the by-products of marijuana smoke, CBD and a meager amount of THC. Hell if you like bowls and you like to smoke resin whaatever but a MAPS/NORML study has shown joints to be very efficient.