Because they don't have a right solution advancing the times to redfine the Informations of the Interface of the microwave.
The example is very good one to explanation how importance of design ist. And why is this microwave interface so complicated? It's really a long Story.
First, Not every function is the same importent to the users. So there musst be a change to show the difference.
Second, the Numbers from 0 to 9 takes a large place, which they don't need to. They did not find a better way to show them.
Third, there is non ideas of user experience und interactions. I did not feel.
So designers musst to redfine all the Informations, and know which level themselves are.
And User research is much importent hier. We need to know that, which funktions ist importent oder frequently used to users. Deeply as more as we can, find the close interaction experiences between the microwave and the users.
Finally give them reasonable position, color, lightness, size and the surrounding relationship.
It's my personal opinion. I try to write more, but my english ist not good.