Because it's likely not a conscious decision for various reasons...
- different teams working on different platforms, so they don't notice that something is different
- different implementation on the different platforms...if its "auto on" on one but not the other (speculation)
- translations between various coding languages
Almost all of the instances we see are Xbone Yes, PS4 No...not situations where the Xbone has a higher level of AF than the PS4 version...and a bunch of those games have been updated to include AF on the PS4 at no performance cost, or at least completely negligible...
That tells me it's not a hardware issue..
Also, even in the instances when the Xbone has AF when the PS4 doesn't, I'm not sure "proper AF" is the word we should be using..as both consoles tend to be on the lower end (2x, 4x) AF instead of 8x or 16x that you would get on the PC side...
Most of the evidence points towards something in the development process..not the hardware