Why has the internet become so pessimistic and political? Because
people are so pessimistic and political.
Has it always been like this, or is it getting worse? Yes. Yes, people have always been like this, and it's getting worse.
Why is it getting worse? I think it's because the gain is too high (think of an audio system developing feedback). Before, when you ran into some opinionated loudmouth who wouldn't listen to anything, you walked away, or you just ignored the guy ranting in the bar and talked to your own group. You ran into them in small numbers and in limited interactions. Now, they have a really big microphone, and they can be heard all over the world.
So they encounter others like themselves much more often now - others that they agree with, and others that they don't. The fraction of what we hear that is conversation between such people has gone way up. That's bad enough, but it makes some people who weren't that way think that that's the way to be heard, and they start acting that way. Even worse, some people are genuinely converted, and start acting that way out of the conviction of their new zealotry.
That may not be the only factor at play. It may not even be a correct analysis. But that's what I think is going on.