While, especially in Draft and not Blind Normals, some people do genuinely go into regular games to improve themselves and treat it like a Ranked match, others go in with a much less determined mindset.
This second group usually goes from Ranked to Normals for easier matches, room for more friends, and an atmosphere where they can make riskier plays or play silly team compositions with their pals.
I don't think it is an excuse, but there is some truth when someone is not doing well: it is a Normal match no matter if all other allies are treating it like competitive, and it won't directly affect your personal scores (KDA, WR) in Ranked matches, which are much more important.
I see why you're frustrated, though. It is an hour of your time devoted to a game you won't win, which can be nerve wracking, but just try to stay positive and improve yourself as much as you can through that game if someone is adamant to troll. Then report.
[slayer-jinx-catface]