I’m genuinely curious: why do you need smoke detectors? I’m asking as an european that has lived without them all their life in more than one country. They are not mandatory here and not even common. What’s the reason to install ther? (I know the reason is to detect smoke, I’m asking the underlying reason behind the need to detect smoke)

Possibly a difference in construction materials. Most stuff in the US is made of wood and other flammable materials. From what I understand, brick/stone based materials are the most common in European buildings.