Lots of reasons. They are cheap and easy to come by. The exception of course, a 340. Toss a rod out the side of a 360, no big loss. Go find another block and start over. Do that will a big block and a replacement may be a little harder to find. This of course only applies to factory parts. The full-on race stuff is a different story. If you have a sb and get beaten by a bb, no body takes notice. On the other hand, beat a bb with a sb, everybody takes notice.