The numbers on cards become a lot harder to read after the coloring wears off of the numbers. Even as a human it can be hard to read them at times depending on the lighting.

Card layouts also vary a lot, even from the same issuers (for example: the Amex PASS).

Reading the mag stripe prevents fraud and therefore is subject to lower fees by CC companies.

Not to mention, having someone take a picture of my credit card would spook me out a little (yes, even if I'm familiar with the product).