was discussing it earlier with a friend, whats the main cause of them wearing away?

 

Most likely cause would probably be the valve head getting to hot due to running lean/preignition which will wear the seat alot faster.
Obviously there are alot more reasons why seats can wear.

 

Poor guides are a common cause. The valves tend to wallow to one side, smacking the seat on an angle instead of smoothly making contact. We invariably reguide when overhauling heads.
Poor quality air filtration doesn't help either of course.