It is possible that you have been very very very unlucky and all the irons you've bought were lemons - just waiting to fail. It could be that you wear them out by ironoing all the time. It could be that you abuse your irons in some way causing them to fail prematurely - it could be but I doubt it.
It may be that the socket you usually plug them in to is defective and while you've been ironing away a loose wire at the back of the scocket has been constantly switching the iron on and off giving it a lifetime's on and off stress in a few months.
The irons we use in our house get medium use from three people's washing and last about two years.
Get your ironing wall socket checked - iron at a different place - from another socket.
You're not pulling at the cable and stressing the connections are you?
Alternatively - give up ironing...
Richard Buxton
September 2008