If Ripple Recommends the Adoption of Its UNL, doesn’t that create a centralized system?

No. The XRP Ledger network operates on an opt-in basis. Each participant directly or indirectly chooses the UNL it wants to cooperate with. If Ripple or any of the other publishers stop operating or act maliciously, participants can change their UNLs to use a list from a different publisher.

Incentives

The primary incentive to run a validator is to protect and preserve the continuous stable operation and evolution of the XRP Ledger. It is the validators who decide upon the evolution of the network, so any business that uses or depends on the XRP Ledger has an inherent incentive to ensure its reliability and stability of it. Validators also earn the goodwill and respect from the community by contributing in this way to the ecosystem.

If you run an XRP Ledger server to participate in the network, the additional cost and effort to run a validator is minimal. This means that additional incentives, such as the mining rewards with Bitcoin, are not necessary. The structure of the UNL has been set up in such a way that for operating a validator, no XRP is received. This is done because the behavior of validators could change if this was the case.

For examples of how incentives can warp validation behavior, read more about miner extractable value (MEV) here.