And yet, all those forums manage to pay their bills with ads. Generally relevant ones, too. And vendors who have a good track record get a lot of word of mouth advertising for free.

I (mostly) lurk on the machinery forum, but I know who I'm going to call when I need new bearings for any of my equipment.

Is the problem that VCs only fund companies that go for broad but shallow appeal so they can make a huge exit with less effort compared to building a useful community, or am I missing something?

I think a big part of it is exit potential, yes. Content plays, especially the sort of niche stuff we're talking about, come with built-in "addressable audience" limits.

There are a few niche forums I've been visiting for a long time. You can build a very loyal audience. And I can even think of ways they could extract more money from me than they do - I think they can make great businesses that provide steady income. But you are never going to see an X000% pop from your investment in a metalwork or sculpture forum.