Because it acts internally on heartworm and intestinal parasites. Flea and tick things that only act on external parasites like fleas and ticks are OTC. Flea and tick things that also act internally on internal parasites require a prescription.
The ones with internal coverage are regulated by the FDA. The ones with external only coverage are regulated by…the EPA? I think? But a different agency.
I would assume that because these things act internally, there are risks that are different than things that only act externally. Maintaining a veterinary client patient relationship is necessary to write a script, and requires a yearly visit. That’s not your vet being unreasonable, but a requirement to maintain their license. Selling you rx dewormers (are you sure they’re rx? Strongid, panacur and drontal are all otc) without a yearly visit is risky in the same way.
I’m not arguing that it isn’t frustrating, and I wish, too, that there were easier avenues, or at least more inexpensive “check in” visits. But there’s a lot of regulation beyond your vet that’s driving this, it’s not “just” them.