why does "folk" need to be so broad as to mean anything anyone wishes?

I don't think anyone said it did, it's just that there are a lot of definitions being tossed around that are more restrictive than either necessary or fair. Actually, even if you take out what most agree is NOT folk (opera, Mozart, the Spice Girls), even that narrows it down past "anything anyone wishes".

I don't expect there ever to be a consensus about this but everyone is going to have a different viewpoint based on preferences and experiences that is going to conflict with somebody else's preferences and experiences, so asking an open-ended question like, "What is folk" is like popping popcorn with the lid off. Chaos ensues. We've done this over and over again: Is blues folk? Is bluegrass folk? Is rap folk? I've been told they are and that they are not, but where I live I can assure you they most certainly are, by qualifiers that most people here seem to think are the most important. So I say "yes" because that's my experience (if not my preference; I can't stand rap). There's a lot of the Folk Revival that I'm pretty sure is pop but that others will swear is folk. I disagree but, OK; obviously it means different things to us.

As my old boss would say, "It's a bit like a wild bull--you can get him in the corral but you may never get the ring in his nose," meaning that you may have to settle for narrowing it down rather than defining it to a T.