You could buy a stock expecting asset appreciation, but the stock itself is a certificate of ownership in a business. Even a derivative of this, like options trading, is a contract to buy or sell or deliver said certificate of ownership in a business. Ultimately it is backed by something of tangible value.
"Predictions markets" are all of the mechanics of trading, but without any underlying value. It is literally gambling wearing Groucho glasses. The bet is simply for the sake of the bet.
There is now effectively little to no difference.
With stocks if I try to do insider trading I run the risk of Jail time. Or if I try to do a Pump and Dump scheme, I go to jail. If I am an person and I "place a bet on polymarket/kalshi" and then go and influence that outcome. It may not be illegal. Since these markets commodify literally any event, its impossible to control the items themselves. So the markets need to be controlled.