Why don't cameras have DOF included in the viewfinder info?

I don't shoot enough to commit to memory all the various DOF values. I shoot manual mode with various lenses (zooms included) and usually "hope" my DOF is OK (e.g., group of people at various distances). Often, I stop down to increase confidence but may unnecessarily sacrifice ISO. I don't want to tell everyone to wait while I open Photo Pills to check the DOF so I likely overcompensate with smaller aperture.

Why don't modern cameras include DOF in the viewfinder? They know the aperture and sensor size. Can they not detect the distance to the focus point? I understand there is a focus magnification feature, but that's in Manual Focus and I want to use Auto Focus. There is also "DOF Preview", but that requires fiddling with the camera as I'm trying to shoot a potentially impatient group or moving scene.

Seems I have to either 1) Shoot more to have this committed to memory, 2) Memorize DOF for some frequently used focal lengths and distances, 3) Use Photo Pills DOF calculator as needed. Or all of the above. Just seems like DOF in the viewfinder might be possible to save time?