I've had the same problem. I don't think it's anything to do with DVD players specifically, just with electronics and consumer products in general. Products are being made more cheaply so they can be sold more cheaply. Durability isn't really something that's the highest priority for either manufacturers or consumers anymore, especially with the rate at which electronic toys become obsolete.

I'm on my third DVD player in 4 years.

On the one hand, anything with moving parts can certainly break down over time, but on the other, nothing sold today should break within two years.

And on yet another hand, my DVD player will be seven years old this April, and has been repaired once. You didn't give us any information on if you'd been buying DVD players at the high end or low end of the market, though.