What Are the Economic Incentives That Discourage Miners/validators from Launching a 51% Attack?

The primary economic disincentive is the massive cost involved, which includes the expense of acquiring the necessary hardware/coins and the operational costs (electricity). An attacker also risks devaluing the very asset they hold or mine by compromising the network's trust, which would cause the price to plummet.

The potential short-term gain from a double-spend is outweighed by the long-term loss of the asset's value and the cost of the attack itself.