I assume that you don’t take the trailer on the highway. I live in PA too and no shop doing state inspections would pass those tires. I have been pi&&ed off more than once when tires were failed because of cracks between the tread ridges that didn’t look bad to me at all. Another excuse to sell a Chinese “trailer use only” tire.
As to why the tires crack has to do with sunlight and ozone degradation of the rubber. Yes, tires used to go for many more years without cracking. The reason for the cracks is formulation changes in the rubber used to make the tires. I’m a retired chemist, although not a rubber chemist, and my guess is that additives that used to be put in the rubber to counter the effects of sunlight and ozone are now banned.
The cheapest things in a rubber formulation for tires are going to be carbon black and silica. The chemicals added to get the rubber to link together (vulcanization) and harden and resist ozone and so on are going to be the most expensive. No wonder that the cheap trailer use only Chinese tires wear out fast and develop cracks. They just scrimp on the chemicals used in well made tires.
