The prescription stuff has different active ingredients. You can buy prescription meds online, it’s just a case of asking your vet to give you the prescription (they normally charge a nominal amount for this) and then you can buy the same stuff online a lot cheaper.
We’re lucky here because the drugs in question aren’t controlled, so I can buy Milbemax and Advantix online without prescription.
We're the same here. I've bought worm/flea/tick control at the vet but it's not with a script and I can buy it from any pet store or online without prescription.
I will admit that, over the last year, I've paid a premium for the wormer because I'm lazy

. The vet sends me a text when it's time to pick up her dose so I just stop on my way home from work, pay for the single dose (stupidly expensive) and give it to her with breakfast the next morning.
We have only treated for fleas/ticks once (in January when she picked up a few ticks from a very remote area). Here, if you're using flea/tick products, you have to use them all year round as it doesn't get cold enough in winter. We're in a low tick area and Ella has never *touch wood* had fleas.