The OP asked why shoes needed to be made from leather they no longer need to be.... they are nicer, and last longer but the synthetic option is now viable for the meat is murder brigade... of which I am definitely not a member
You can make shoes out or rubber, fabric, fur, plastic, string or lots of other things.
Leather is most comfortable, as it "gives". Plastic or vinyl shoes can be very sweaty, even when your feet are feeling cold. Also, you can re-colour leather shoes with shoe-dye when they are getting a bit tired-looking. And you can make a handbag match exactly with the same dye.