They are not the only highlights, especially when we talk about Bloodborne, DS3 and Demon's/Remake, or Elden Ring.
One of the biggest highlights are also the areas you explore. These games have so much atmosphere it's easy to get sucked-in, versus a lot of other games. The bosses are like the icing on the cake.
I feel if the areas aren't interesting, why would you explore and find a boss in the first place? I feel both are intertwined and From has been excellent at getting us interested in their scenery and bosses. You want boss rush? The Chalices in BB are like a real boss rush, especially if you know the configurations and how they work. You just open levers until that one main door opens, but sometimes the Chalices get deep and you have to do some fucking running around in order to get that Main Door open.