My cat is stuck wearing a cone.

A year ago my cat had an IV in his leg at the vets and when he came back, his paw was swollen and ever since he has been constantly biting the site where the IV went in and causing it to bleed. We've taken him back to the vets multiple times but they say they don't know why this is happening, they've tried to link it to stress, and have tried steroids and many different sprays but nothing has changed. Having the cone on stops him chewing his leg but I want to help and understand why this is happening.