User tangoshukudai on Reddit:

It is actually a funny story. When they were prototyping the iPad, they ported the iOS calc over, but it was just stretched to fit the screen. It was there all the way from the beginning of the prototypes and was just assumed by everyone at apple that it was going to be shipped that way. A month before the release, Steve Jobs calls Scott Forstall into his office and says to him, “where is the new design for the calculator? This looks awful” He said, “what new design?” This is what we are shipping with. Steve said, “no, pull it we can’t ship that”. Scott fought for it to stay in, but he knew he had to get their UI team involved to design a new look for the calculator but there was no way they could do it in that short time frame, so they just scrapped it. It has been such low priority since then that no one cares to work on it since there is more important things to work on. (Source: I worked at Apple)

They should have just licensed PCalc [App Store].