A) Riot can't really go ahead and issue punishments based on forum posts. If they ever did it, even just once, people would go ahead clog up the forum with shit like this, because most people think their case is somehow special. As a result, reporting people on the forums doesn't even usually lead to any punishment.
Sidenote: Even if Riot would choose to take action, they probably wouldn't need you to tell them the name of the player that did it. Your description would most likely be enough in almost all cases.
B) People like to jump onto bandwagons. Take a look at the Tyler case. I'd say about 90% of typical forum visitors felt very strongly about the case... even though probably only very few really watched his videos or generally informed themselves.
Even if it doesn't usually end up like that, naming and shaming can lead to extreme witch hunts.
C) You can convince a lot of people that something happened just by saying it did. You can convince even more people with a few screenshots (that can fairly easily be created with photoshop). You can convince even more people if you have smurfs and upvote/comment on your thread a lot.
And while most people won't do that, the typical forum user doesn't have any way of actually verifying your story. And while that may not sound like a large problem, it is a huge problem when combined with the previous post.
And ultimately:
D) Because there's a better way. Reports, or, if you absolutely must have somebody look at it manually, you can submit a support ticket.
Neither of these have the aforementioned problems, because Riot will act on support tickets and reports (even if you feel they don't), Riot can look at things objectively without peer pressure, and even more importantly: Riot can look at things objectively and verify your story.
tl;dr:
A) Riot won't punish people based on reports on the forums.
B) Naming and shaming leads to witch hunts.
C) Nobody can actually verify your story.
D) There's already a way to get Riot to look into your case, that doesn't have any of the aforementioned side-effects.