I guess that’s because our minds (and animal minds as well) are always aware of the pace of repetitive events. If something is off, the anxiety alarm rings. One old book on the brain machinery described an example of a cat that was relaxing near the metronome and became alert when it was suddenly stopped. Unpredictable delays are disturbing, because a mispredicted event means you may be in a dangerous situation and have to recalibrate now.
I think the explanation may be even more low level than that. Iirc, even with a single neuron (or maybe if it was very small clusters, sorry recollection is a bit hazy) you can see that it learns to tune out a repetitive signal.
