Because we want to generate millions of extra pageviews by making you click through each one, one at a time.
No, really, it all depends on the options you choose, either in your profile as your default or anytime you're anywhere in the forum. There are four modes: flat topic/flat message, flat topic/tree message, tree topic/tree message, and tree topic/flat message. You can select your default in your profile (it's at the bottom), or just click on the button which says Tree View (if you're in Flat View) or Flat View (if you're in Tree View).
Flat view of topics shows only the initial topic in the thread, along with the number of replies and views and the last message (poster and date/time); tree view does not show the number of replies as a number, but instead shows the hierarchy of all replies in indented format down the page (the "tree"), along with who posted and when (but number of views is not shown at all). Likewise in messages, flat view resembles the old Rec Pit in showing up to 40 posts per page (useful if you want to read or scan them all in the order in which they were posted), while tree view shows only the current post at the top and the tree of replies underneath (useful for quickly scanning for who posted in the thread or for seeing the posts sequenced according to the post (or reply) being replied to, rather than the order of post time-wise--IOW, a post which might appear last in the flat view because it was posted last, might appear third in the tree because it was actually in reply to the second post in the tree).
The upshot is you have far more control over how you view the posts than you ever did in the Rec Pit.