Because it is early days for Android still (don't forget Android is less than 5 years old), and manufactures are still doing wacky things, and people are still uncovering bugs and edge cases.
In the early days of MS-DOS compatibility was a big problem too. Magazines used to use MS Flight Simultor to test how compatible PCs were - plenty failed that.
There is a difference though: The IBM PC was the undisputed gold standard for compatibility. Software typically listed the hardware requirements as "IBM PC or 100% compatible". (Some non-IBM-compatible DOS hardware was sold, but it died out pretty quickly).
It doesn't look like there any similar gold standard is emerging for phones.
In the early days of MS-DOS compatibility was a big problem too. Magazines used to use MS Flight Simultor to test how compatible PCs were - plenty failed that.