The reason was that IBM bought the rest of the chips "as they were" from Intel, so all the logic that they made themselves ended in the in-house keyboard contro

The reason was that IBM bought the rest of the chips "as they were" from Intel, so all the logic that they made themselves ended in the in-house keyboard controller. x86 had an external interrupt line for reset, and an io pin in the keyboard controller was tied to this. That was a reasonable way to get true hard reset back then.