Because sometimes it is better to keep a large number of people occupied with one specific goal instead of trying to convince them to do something else. This ultimately boils down to people, most of them are very smart and capable and could do any number of other things, but basically want to work on fundamental particle physics. You couldn't easily convince them to build a telescope instead (a totally different set of people finds that fascinating) or any of the other things you listed. Besides there are (smaller) collaborations that work on all of these other things you mentioned as well.

Furthermore there is good motivation to keep nuclear / particle physics research as centrally organised as possible because the people working on it would be equally capable of producing nuclear weapons. For example a good number of the people working with CERN in France sit right across the Institute that developed their nuclear weapons.