As indicated on the previous posts there was probably a bad batch of O2 sensors that went into the cars in '02-'03. I had my '95 ES for 9 years and nearly 100K before the first O2 sensor went bad. There were three on that car and I only had to replace one.
Not 100% up on it, but I would also think that emissions standards on newer vehicles are more stringent than older ones and its possible that O2 sensors have gone through some redesign and tuning since '95 making them more sensitive to these new requirements.
As to what causes them to go bad... over time I would think that enough deposits from the exhaust gasses build up on them, excessive heat, continuous cycles of cold/hot/cold/hot, etc will certainly wear on any electrical component (not to mention cause the wires to turn extremely brittle).
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Because, I have 97 ES300 with 142,000. NOOOO Problem with O2 sensor.
But i use 93 Octane.