Why are water heaters so unreliable?

[David J.]

I have had a water heater fail (or need parts repaired) in every property I have owned.  Seems like by now, we should have the ability to produce a water heater that can simply work without issue for 20 years. 

These things have to be the least reliable thing in a home,.. or maybe in life.  (perhaps i'm just frustrated)

I am no thermodynamics engineer but isn't it just a tank of water being heated by a flame?