If they won’t sell it to you without confirming with the vet, then that means it does have something in it that can’t be sold without restrictions. Otherwise, capitalism would do what it does best, and they’d take your money. So there is something in that food that they’re not legally allowed to sell you without approval from a vet. I can’t tell you what that ingredient is, since I’m not a pharmacologist or a vet.

I would, however, recommend that you enquire with your vet about why you need to take your cat in for a repeat prescription. One of my cats has been on and off various antibiotics over the last 18 months (he has an incurable viral infection that leads to persistent secondary bacterial infections), and when symptoms haven’t changed and a course of antibiotics just needs to be continued for another few weeks, the vet does a consultation over the phone and then issues the prescription. I then pick it up from reception later that day without having to take my cat in. It’s only when his symptoms have changed or an antibiotic isn’t working that he needs to actually go in for a visit.