Why are OEM tires crap?

With our first Ford Escape, a leased 2005 FWD XLT, the tires that came with it (Continental ContiTrac?) lasted almost 3 years before needing replaced.

On our purchased 2008 4WD XLT, just over a year old and with less than 22,000kms on it, we had to replace all four tires this morning... ~$700. :(

For the last month or so, on mostly smooth roads, we could hear a light "thup" sound that would speed up or slow down, depending on how fast we were driving.

I suspected the tires. We took it to the dealership and they confirmed that it was indeed the tires and they openly admitted that it's not the first time they've seen that happen with the original tires. Unfortunately for us, there's no warranty on OEM tires.

I've always heard that OEM tires on all makes of cars are generally crap, but this is the first time that we actually encountered a problem.

The guy at Canadian Tire said that OEM tires generally have a lower quality rating than the exact same tires if they were bought new.

Why do tire manufacturers willingly sell poor quality tires to car manufacturers? As a consumer, I might now label all tires from that manufacturer as being crap, even if they're not, because they sold a sub-standard tire at a cheaper cost to the car manufacturer. They're only hurting themselves in the long run.