[RoyJackson]

I wonder why the lawn and garden engines wear out so much sooner?

Lack of use is the biggest abuse! Sitting is real hard on a motor. Most lawn and garden stuff sits most of the time.

So do most tractors!!

Smaller displacement engines have to rev higher to make power. This does accelerate wear, somewhat. Also, diesel engines are built heavier then an equivalent gasoline engine. Most lawn and garden equipment do use single or two cylinder gasoline engines.
However, the point made about setting is valid, especially if condensation builds up inside the engine (such as short trips, etc.). There was a thread a while back on this...