Because you’re getting a product that you know isn’t a cheap knockoff that will burn your house down, and you know it will charge your phone at the fastest speed it’s capable of.
You can of course get the same experience buying third party, but then you have to spend time doing research on which one to buy for your device, and the reputable third party brands can cost just as much as the Apple ones anyway.
and you know it will charge your phone at the fastest speed it’s capable of
Are you really advocating for buying an Apple-branded USB-C cable?
Yeah, why not? A quick look at Best Buy and I can see that the Apple USB-C cable is $15.99 and the cheapest reputable third party USB-charger is $13.99. You save a whopping $2.
So if you’re a deal-oriented shopper you’re probably not even going to buy from a reputable third party, you’ll probably go with the $6 one from the gas station of dubious quality. And you’ll probably be fine. Or maybe after 3 months it causes a short and burns your house down. Best $10 you ever saved.
Or you can take literally all of the guesswork out of it and just go with whatever manufacturers cable, spend the extra $10 on a cable that will last you years. The point isn’t buying something Apple branded, they don’t even brand it physically. The point is to just buy something guaranteed to work.