Because not many people care (yet). Of course font should always be defined in physical units (be it pt, inch or mm). Pixels are irelevant. When you are printing a text, you also don't care about how many pixels are being printed - your printed page will look the same on any printer, you can print a ruler. But somehow software industry has ignored this for decades. Why? I don't know. Probably inorance, bad cooperation amd lack of standards.