Most prop firms, for even their funded traders (who pass the challenge), they never send orders to any liquidity providers.
What this means is, even if you get funded after passing the challenge, you are essentially trading against that prop firm, because that prop firm is the market maker.
You are essentially trading a fake market. The broker can set any conditions they want, widen the spreads, bad fills etc.
Will the prop firm have any incentive to see you win, when they are on your opposite side? You can decide that.
If they did send orders for funded traders to actual market via LPs, you could make an argument that they are incentivized to see you win & share the spit, because they are not the counter party. In which case they will actually be funding winning traders.
But are they really?