> Why are these big insider bets being placed within hours of the event actually occurring?... It also makes them largely useless

They're not, usually. The more the outcome seems unlikely at first, the more you're incentivized to place your big bet earlier, when the odds seem worse, because you'll make a ton more money.

When these bets are placed only a few hours beforehand, that's often because the actual decision hasn't actually been made until then.

And there are plenty of areas where having notice of an hour or two is still hugely vulnerable.