Board Game Arena

When playing solo mode the game seems to end quickly. Is this because it's just a training mode and only so many years can be played?

Yes. In solo mode you have to reach the 20 mark or higher before the end of the 7th year (on normal difficulty). If you have mainly played 3 or 4 player it might seem like a much shorter game but in reality it plays over about the same number of years as a game with average skilled players would.