My beloved iPhone SE 3 is not compatible. As for other people, well, it is quite personal, some people prefer to just not use it at all. I would leave it on and use it just when I needed to summarize a text or something like that, but I get that more minimalistic users prefer to turn it off.
As for Apple itself, maybe they preferred to leave it out of other languages to focus on English during the first months and polish the experience on it. Just look at some funny/embarrassing mistakes that notification summaries produced. On the main language at Apple. In other, less known languages at Apple it could’ve been way worse, so I understand they are delivering the feature to different languages step by step, and beginning only with English. Hopefully, one of the most spoken languages (along with English and Chinese) such as Spanish will be included on the first AI expansion rounds.