Reason for automatic box is that Grenadiers are aimed at the NGO type market in direct competition with The Landcruiser - there is not a lot of difference in price, but they are both built to last a long long time. They will often be driven by drivers not well trained in off road driving and and auto box saves huge wear and tear of clutches and drivetrains. The hydraulic torque converter smooths out throttle inputs etc. the ZF 8 speed box is the same used in many big 4x4. I have one in my Toareg. I did view manual as prerequisite for offroad driving. But an a auto box really does make life very easy. And with a low box you can drive very easily and accurately without riding a clutch.
Yes they are expensive, but in markets like Australia they are not that expensive compared to Landcruisers of similar spec. All the running gear and suspension is hugely tough and will probably hundreds of thousands on rough roads. ZF Box and BMW straight six provides a tough powertrain.
Most who buy them will either be fleet / commercial buyers either on finance packages or on balance sheet. Long life and high residuals will be key.
If you want cheap buy a Great Wall.