They do try and there are many cities in the US implementing their ideals. Their major problem is that the people who live in the areas they want to "improve" don't want their "improvements", which is why they came up with conspiracy theories like Big Oil and Big Car brainwashing everybody to prefer houses and cars. A typical suburban resident has no problem building a trail system but has an enormous problem with 50% of the car lanes being taken away and given to a handful of cyclists. Urbanists don't want to improve transit/biking access; they want to destroy the ability to drive. This is
the key point that a lot of the people in this thread are missing.
No greater example can be shown than the seethe they have for truly multi-modal infrastructure:
Here's a nice small intersection in Japan with bike lanes and a pedestrian bridge:
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You would think something like this is exactly what the urbanists would want, but you'd be wrong.
They actually hate this:
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Here's a similar bridge in China, but in a much bigger intersection:
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There are massive pedestrian plazas on both sides of the street, a two-way bike lane on the left side of the street that is shaded by trees, and a nice bridge to allow pedestrians to cross the major road without waiting for the lights to change.
What do reddit urbanists think about it?
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A second picture of the same bridge up close (Chinese redditors appear to be quite proud of it):
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Notice the common thread throughout all of these comments: it's not enough to improve non-car transportation (I doubt anyone can say with a straight face that Chinese and Japanese cities don't have good transit), you have to actively make it difficult to drive. Their ideal city is the medieval European city center, and they hate anything that doesn't look like that.