The Pixel is the closest. The reason you don't see long term support on Android is because of Qualcomm. Qualcomm wants manufacturers to build on new chips, so they deprecate older chips and stop support. Most manufacturers don't want to hire kernel and hardware devs. Samsung can pull off longer support because of Exynos and they have a lot of inhouse expertise to extend support on old Qualcomm chips. It's all money.…
The reason you don't see long term support on Android is the same reason Linux failed in the desktop: the Linux kernel developers want everything mainlined and are not interested in keeping a stable kernel API for drivers, but in the real world, drivers are closed source.
(1) Linux didn't fail, the desktop is already a joy to use imho. It is just waiting till it reaches critical mass, then the Year of the Linux Desktop will be nigh.