Threaded Mode

Why don't modern forums and message boards have threaded mode?

Everything is linear mode, this causes threads to go to shit because some guy replied to another guy. With threaded mode you wouldn't have to see useless derailing replies.

Tree mode

Note: Branches of replies are not visible unless you open them.

Interesting post
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First reply
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Shit post ──── 100 replies to this troll that would've derailed a linear thread (not visible)
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Continuing conversation of OP's post
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Interesting comment ────(manually open this branch)──── Reply to interesting comment
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  |                                                     More posts continuing this line of thought...
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Continuing conversation...

&ct.