Dealers simply put the cheapest tires they can find on cars they're selling. It's all about dollars and cents. Perhaps some of the nicer dealers will put a reputable brand on, but that seems to be a rarity in my experience.
Go to your nearest certified used car lot (that sells turned in leases) and you will see what I'm talking about. These days, I'm lucky if a used car has Kumhos or Hankooks on them. Usually it's Milestar, Primewell, Ironman, Linglong, (insert east asian company here), or some brand I've never even heard of.
When we recently bought our used van which needed tires (28k on original tires.) I asked the dealer to just give us a few hundred off the price because I don't want the junk tires the dealer would put on. He gives me the line about how they can get them cheaper than I can, but that he will put on a good "domestic" brand. I wound up with Travelstar tires which are most definitely foreign. Won't make the mistake of letting dealers put tires on for me again; either they give me money off or I walk. I expressed my displeasure but didn't force the issue, but that experience will be reflected in the online reviews I give. Luckily the tires have a 340 treadwear rating, so I'll probably be replacing them sooner than later!