[scubbo]

Because making a prediction publically can affect the event itself. Either in a positive feedback loop (Elon Musk tweets "Gamestonk" => Gamestop stock goes up) or negatively (in any hypothetical situation where an adversary wants to frustrate you, allowing them to know your prediction also allows them to work against you).

[jmcqk6]

Having secret predictions are also problematic.

Imma going to go and make a whole bunch of predictions, wait a year, and then only reveal the ones that actually came true.

Now I'm a genius, right?

This is basically similar to an old common scam, where you would mail 100 people, and 50 would say 'buy this stock it's awesome', and another 50 would say 'sell this stock, it stinks', and to 50 of those people, you're going to look like a genius, and to the others, you're just another scrub.

Now I could go and make several predictions for the top 100 stocks, planning for every possible move, and then as they come true, start to reveal those predictions and BAM, I'm a fucking genius, give me all your money, and I'll invest it for you.

[SamBam]

I think these always depend on generating the hash and publicizing it ahead of time. Otherwise yes, it's worthless. But I think most people realize that.