Originally Posted by dcarver
Why do manufacturers put tires on that are not rated for the GVWR?
Not required. What is required is to have enough weight rating to handle the combined GAWR of the trailer. As you've now realized, hitch weight (tongue or kingpin weight) is not hauled by the trailer tires, but by the tow vehicle's tires.
Example. My old Keystone Sprinter 25RKS 5er has GVWR of 7,900 pounds. But combined axle ratings are only 7,000 pounds. If I load the trailer to 7,000 pounds on the trailer axles, with 15% pin weight I'll have GVW of 8,050 pounds, or overloaded over the GVWR of the trailer. If I don't exceed 7,000 pounds on the trailer axles, then I won't exceed the GVWR of the trailer, and my trailer tires rated for 1,820 max pounds each (7,280 gross) are enough for my trailer.
But I blew out two of those tires on the first long trip, so when I got home I changed out all the ST205R15C trailer tires (and wheels) for ST225/75R15D, which have 2,540 pounds weight capacity, or 10,160 gross. That's plenty for my 7,900 pound trailer, and I never had any more trailer tire problems for the next 12 years towing that trailer about 100,000 miles.
My 2012 Skyline Joey TT had ST205/75R14C trailer tires, rated for 1,760 each or 7,040 pounds gross trailer weight. Since that TT has GVWR of only 5,600 pounds, those small 14" trailer tires are all I really needed. I've towed thousands of miles with those stock trailer tires with no problems. But I prefer to have a little more fudge factor, so recently I replaced all 5 with ST215/75R14C with 1,870 weight capacity, or 7,480 max trailer axle weight. With 5,600 GVWR of my TT, those slightly bigger trailer tires gives me a huge fudge factor.