Pinned topics are automatically unpinned once you read them and get to the very bottom of them. The thinking here is that they take up valuable space at the top of the page, especially if their are several pinned topics/threads, "bumping" more active topics down the page.
On vBulletin, our old platform, similar "sticky posts" sat there basically forever, doing just that.
In their current form, having them become automatically unpinned doesn't help you, if you're looking for an easy to find reference topic/thread. Have you checked out the bookmark feature? You can do this from any post, using the Bookmark feature in the action bar, and then get a list of all of your bookmarks by clicking on your avatar in the top right of the page, and then on the bookmark icon (right next to the Messages/envelope icon).