Equities are underlying collateral. Prediction markets are literally just betting on an outcome, no underlying asset exists.
Collateral is not uncommon in gambling (e.g. pink slips). That does not seem to distinguish gambling from speculating.
What collateral is underlying the massive, state-sponsored, Spanish lottery ticket and scratch off racket?
Prediction Markets act the same way as Gambling Exchange - the assets are denominated as both sides of the book minus the spread.
https://www.betfair.com/exchange/plus/