Actually they have. It's already a surprised that it lasted as long as it did. Imagine, the sensor has to be sensitive enough to compare O2 levels inside and outside of the exhaust and at the same time must withstand between 700-1000 degrees C for a few years worth of driving.

So if it fails after 4 years, I consider it a pretty damn strong component. Even stainless steel extractors and exhaust manifolds can fail earlier than the O2 sensor.