All round the world tickets I could find so far did not extend 1 year. Is there any airline or alliance that offers round the world tickets for more than a year?
It is an IATA recommendation, and airlines tend to go with the minimum recommended in case the policy is in their favor, and opposite is true when the recommendation is against them:
3.2 PERIOD OF VALIDITY
A ticket is valid for carriage for one year from the date of commencement of travel or if no portion of the ticket is used, from the date of issue thereof, except as otherwise provided in the ticket, these Conditions or Carrier’s Regulations.
There are conditions in which the validity is extended. Basically when it is the airline's mistake, such as cancelling a flight or so.
Many airlines include this in their "General Conditions of Carriage & Tariffs", some types of tickets have less validity, usually the discounted tickets (~3 months).
Source: Recommended practice 1724