Fans are too close to the pitch, or maybe too stupid to understand the concept 🙂
Either way ball goes into the stands, player grabs another ball from a cone to take a throw in, and 10 seconds later the original ball gets launched into the pitch from the crowd and play has to get stopped whilst it’s cleared off. Ends up wasting even more time. Also potentially open to abuse by fans hanging onto the ball until an opposition attack starts and then returning it onto the pitch to stop the play.
I just don’t think the benefits outweigh the negatives to be honest.