Could you define what you mean by "so often?"

There are certainly ships that are intentionally destroyed in what are called "sinking exercises," in the U.S. navy abbreviated as "SINKEX," which are meant to study the real-world effects of various types of ordnance against ships. But far more ships are scrapped than sunk.

If you have any particular examples in mind, I can try to pull some resources together for you. Keep in mind that events after 1995 are not appropriate for discussion on this sub.

Correct, it is very labor intensive and dangerous to dismantle a ship for scrap.

[This Baltimore sun essay (it won a pulitzer) shows how dangerous to the environment and workers it is to scrap a ship.] (http://www.baltimoresun.com/news/maryland/history/bal-pulitzer-shipbreakers-gallery-storygallery.html)