I think it's a result of how quickly the number of comparisons increase. If I have 3 options, A, B and C I need to make 3 comparisons. A vs B, A vs C, B vs C. I

I think it's a result of how quickly the number of comparisons increase. If I have 3 options, A, B and C I need to make 3 comparisons. A vs B, A vs C, B vs C. If I have 4 options I need to make 6, for 5 options, 10 comparisons and so on. If we assume that people can hold about 7 +/- 2 items in short term memory, you can see why the decision process becomes mentally tiring after 3 or at most 4 items.