As there is no variation with the machine, every ball is in the same spot at the same pace, the batsman can pretty much switch their brain off, no thought, no having to react to where the ball is, as they know they can just play the same shot every single time and the ball will be in the right place, but when you introduce the actual bowler and they have to react, judge the pace, length, direction, and then play their shot they struggle because they’ve got used to the mindless repetition of the machine.

I know it’s not exactly the same, but to me it seems very similar. If you’re simply dumping a bucket of balls out and chipping every one from the same spot then you don’t have to think, you don’t need to actually put in the effort to decide how you need to hit the ball and so when you are put in a position where you’re having to make those decisions it doesn’t come as naturally as it should.