I guess it is because on the Macbook, the "Taptic Engine" (i.e. vibration motor) is directly under the glass of the trackpad, so it's actually gently moving the

I guess it is because on the Macbook, the "Taptic Engine" (i.e. vibration motor) is directly under the glass of the trackpad, so it's actually gently moving the glass, which feels a bit like you've pressed it. On the iPhone, it is the whole phone that is vibrating rather than just glass, so it feels less like you are directly interacting with the glass, and rather that the phone is responding to your touch. I have to admit, I was hoping it would feel more like the Macbook Force Touch too, but I guess it's just not possible to build it so the vibration directly affects only the glass with the current design.