It is either not being offered in depth by anyone market-maker (part of the answer given the relatively small revenue opportunity) or it is being offered by people who aren't sophisticated enough.

Bookmakers offer markets on events where someone can know the outcome. The difference is that they have tools to prevent adverse selection.

Prediction markets offer none of those protections so the market structure is going to end up being very different (which is already happening, revenue opportunity from politics isn't huge). There are other examples of this around latency arb, market is going to be very different.

Also, I will point out that most insiders are probably going to be losing money too. All that you ever read is the final outcome, you don't read the stuff that happens before. Politics is, generally, not a good market because the actual event is driven by decisions made by people. Election markets are fine but political event markets are not good, even if you have inside information.