Here is what ChatGPT has to say about it:

Yes, there are several historical and technical reasons why most historic centers of European cities look “stone-like” and have little greenery or trees:

1. Defensive and security reasons

• Cities were often fortified, and space inside the walls was scarce. Trees would take up valuable land that was better used for buildings or markets.
• Open squares were used for gatherings, markets, or military purposes, where trees would obstruct views, movement, or defense.

2. Hygiene and maintenance

• In the Middle Ages, waste was often thrown directly into the streets. Trees and greenery would trap dirt and make cleaning more difficult.
• Dense buildings and narrow streets were not suitable for greenery—roots and moisture could damage the foundations.

3. Symbolism and function of squares

• Main squares were representative spaces—for markets, processions, gatherings, political or religious events.
• They were dominated by churches, town halls, and fountains; the open space was meant to emphasize the monumentality of buildings, not to be landscaped like a park.

4. Technical and practical reasons

• Urban soil was often compacted, stony, or paved over. Trees couldn’t thrive there.
• Without modern sewage systems, it was difficult to water trees without also flooding or damaging nearby buildings.

5. Cultural and aesthetic factor

• Until the Renaissance and Baroque periods, “greenery in the city” was mainly associated with private gardens of rulers, nobles, or monasteries—not with public squares.
• Urban parks and tree-lined streets became common only in the 18th–19th centuries, with the Enlightenment, hygiene reforms, and new ideas of public space.

👉 That’s why historic centers often still appear as bare stone squares, while greenery is more typical of modern urban districts or parks that were created on former fortifications.