Why have so many major sites stopped having chat rooms?

Way too much verbal abuse.

One guess of mine is men posing as teens and/or trying to have a sex talk or even hook up with a minor. (think To Catch A Predator on NBC)

Supposedly to assist in stopping pedophiles. Supposedly.
MSN Chat was the best and no other has come close to matching it. Yahoo sucks bigtime ~ it's worse than it ever was.
Glad I found Answerbag!

It's not pedophiles, because all anyone's seen here is the hype, and not the actual statistics, and it's not abusive teenagers, because those teens pay good money to be savages. It's spam and hacks. Spambots are everywhere, even AB, and there's no such thing as perfect code, especially on such a small-scale project as chat and forum deployment. They're also being quickly obsolesced by local chat clients and a/v chat. Chat rooms are a relic of the days when if you were getting online, you were using AOL and prayed nobody called you those 15 minutes you were marveling at usenet, email and chatrooms.

I can get to know people are not?

I just log off from a chat room just now, and then I came to Answerbag.com. Frankly, I was disgusted at the teen chat rooms. There are many verbal abuse on there. When I was at the chat rooms, there were people who were angry, and immature. Some told me to "fuck off", "Kill yourself" and "nobody likes you, you little cun&". I was just arriving and saying hi, I wasn't looking for an argument or any insults flying at me. There were teens there that told me to "log off" or "get out of here" and " we don't need you here." I can tell why some of those teens killed themselves. It's not a stupid thing for a teen to kill his/her self over the chat rooms. I can understand why.

one word

pedophiles

Liabilities.