First of all, no "excuse" is needed, since vBulletin is a completely legitimate choice.
In any case, you are welcome to develop a PHPBB site that does everything this site does. The requirements, currently, would be that it can manage over half-a-million user accounts with complex permissions levels, can accomodate 15,000 simultaneous users (we haven't hit that yet, but will after the Hardy release hits the headlines), can remain online during large-scale database operations, and can do all of this while running on two middle-grade servers.
Oh, and: you will have to do all of this in your spare time, for free. Let us know what you come up with.
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