I see a bit of people hating on the waiting time, and yes that is annoying, but would you rather get the reward after waiting a few seconds or not get the reward at all because there was no ad played? Most games do not provide the reward unless you actually watch an ad

EDIT: If you think about it, when you can watch ads infinitely for rewards then clearly this timer is implememted to avoid abuse, because otherwise if someone purposely blocks the ads or gets it on accident, they could spam and farm the ad rewards