Because the system needs a stable core to function at all, and that requires that witnesses not only be paid but paid enough to always maintain stable infrastructure and contribute enough of their time to necessary 'soft' functions, but also enough so that concern of losing a witness slot is a meaningful incentive to remain a good actor (if you are doing it break-even or at a loss, who cares if you get voted out once you have messed with the chain, possibly for personal profit or paid by someone else who profits).

Without a stable functioning and secure chain you can't even conduct an SPS vote. For example, witnesses could tamper with the vote by censoring transactions, or punishing accounts which vote the "wrong" way.

A DPoS chain depends fundamentally on the competence and integrity of its witnesses Putting that at risk puts the entire system at risk.