Why isn't the touchscreen pressure reading being put into use by Android?
Don't run to conclusions early - some Huawei/Meizu/ZTE flagships has it, sometimes referred to as "3D Press" or other names.

Ultimately the feature is worthless - it requires 3rd-party apps to integrate well, but since Android doesn't have a native implementation of it (yet), there's no way apps could adapt to all the proprietary standards raised by different manufacturers. The app support situation and the sales of the above phones proves the point.

This situation is quite similar to what fingerprint scanners have experienced on Android before Marshmallow, except that it's of even fewer actual use than it.