3) Private equity, big companies and venture firms bought all the small companies with good moats?

That's my bet as the single biggest reason.

It's worth considering that investing knowledge may be similar to science and engineering knowledge, in that it's cumulative within society (and ever-easier to access). So just like it's harder to invent a better mousetrap or find a better fundamental physics theory than it used to be, it's harder to find mispricings that others haven't also seen. Not impossible, but harder. The switch to passive investing suggests many have given up, although in the extreme that can't work for everyone, right? (There's an ETF called NIXT that just invests in companies removed from indexes. But that's a pretty small universe.) I kinda think allocations between broad asset classes are just about all that's left to exploit, because there's less known about those value comparisons, and the payoffs are over longer timescales. But who knows?