Numberphile did a (https://www.youtube.com/watch?v=kCSzjExvbTQ) on this very question a while back. The short answer is that when phone companies were moving from rotary to push button phones, they came up with a bunch of different possible layouts, the calculator layout included, and asked people which ones they like best.
Watch the video though, it’s great. Some of the alternative layouts that they considered are crazy to think about.
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