In F# (the most widely used ML?) expressions can be much more complicated than that, incorporating loops, conditionals etc.

To illustrate:

    let x = 
      let mutable maxScore = 0
      
      for item in items do
        if item.Score > maxScore then
          maxScore <- item.Score

      maxScore

Compared to:

    const x = 
      (() => {
        let maxScore = 0;
        
        for (const item of items) {
          if (item.score > maxScore) {
            maxScore = item.Score;
          }
        }

        return maxScore;
      })();

Not necessarily the best way to write this (you would probably use a sequence library) but hopefully conveys the idea.

Extension methods are definitely useful in OOP languages. However, a much cleaner language design is to remove the need for a "class" altogether and just have free-functions, function composition and a pipe operator.

None of this is something you can't simulate in OOP / procedural languages, it's just much more clunky.