Yet they did, in the person of David Em: https://ohiostate.pressbooks.pub/graphicshistory/chapter/9-6...

> Why must they have?

Because PARC was an intentionally multi-disciplinary group working on the earliest incarnations of visual computing.

Also, no one is disputing that Susan Kare is highly accomplished and hugely influential.

Because any time you say some is "the first", you obviously looked at some sources to find who was the earliest one fitting some criteria. Relaxing the criteria and/or looking deeper is very likely to find someone else. But you can never win an argument either way, because the criteria are arbitrary. So many disputes boil down to definitions.