Also, compression doesn't lessen the effect of color grading.

Most people won't notice the difference that is true but this is true even for real cinema. Most people don't notice or care about things such as shooting on real film, aspect ratio etc.

The why is that people who create care about these things. Color grading and post shot creative manipulation, these things benefit from extra data. If I can't recover shadows or pushing white balance results in banding, those things are going to still be visible on a phone by myself and the small amount of people who do care.