Guardrails are really ugly, and have been shown to increase traffic speeds -- they make drivers feel safer, since they think they no longer need to look for pedestrians.

I think you might be assuming the aim is to maximize traffic flow, or similar. That's not necessarily the aim of a European city's traffic planners -- they may prioritize pedestrian or cyclist safety, child safety or play space, reducing noise, improving bus/tram speeds, or maintaining a historic appearance. This might be for a single road, a small area, or a whole district.

Several of these can reduce overall traffic. If parents don't think it's safe for their children to walk to school, they might drive them instead -- increasing traffic at the worst time for other children. Similarly for private vehicles obstructing buses and trams.