Try finding a hack for it at phpBBhacks.com. And, as to your question why phpBB doesn't have it is because phpBB hasn't had any development since 2001 (active) and the new version that they are working on is still a dream. :p You can't compare vB to phpBB.
Yes, vBulletin allows infinite subforums. However, for the sake of site usability and navigation, it is not a good idea to go very many levels deep, because members will just get frustrated trying to find the right place to post.
A am using the categories hierarchy mod on my webmaster forums. I am using phpBB. It's a great mod for sure, although it kinda takes some time to install. I think there should be only 1 level of subforums, since otherwise people would just go nuts trying to use the forums :D