I know that the emini SP500 futures contract (ES) is used for many reasons. One of these reason is hedging. From what I understood, hedging should widen the spread between spot and futures prices.
Is the arbitrage the only glue that keeps attached futures and cash market, that's all that matters, or is there something else?

Yes, but I feel this is like saying it is only gravity that keeps our feet on the ground.

See spot future parity.