For shooting portraits, why is 50mm preferable? For example with an APS-C sensor, why is it not ok to take photos with 35mm equivalent and simply crop them (to 50mm equivalent)? or take photos closer to the subject?
What's the difference?
Notice the different perspectives in the image above, for lenses of differing focal lengths, caused by the distance between the camera and subject having to be changed - to keep the ratio of subject size to image size similar in each example.
Cropping a wide angle image to a longer equivalent will not remove this effect.
Moving closer, with a wider lens, will introduce more of this effect.
Cropping will also not affect the DoF in the image - where changing the lens and shooting position probably would have.
The original image (which is not mine) can be found here.
you question has one premise and 2 questions to answer:
P1. Premise: Why prefer 50mm?
We dont. 50mm on crop camera is just about a bit too short. Optimal is 85mm (crop) and 135mm (FF). A good range to experiment within (for beauty - and not fucked up funny perspectives) is 60-135mm (crop) and 85-200mm (FF).
Q1. digital zoom (cropping to simulate the focal length)
It's a nice workaround, but you will destroy your resolution and better optical properties and DOF:
Q2. Go closer instead
The reason there is a focal length recommendation is to get a certain perspective that is flattering. So getting closer is the worst thing you can do.