For some people it's the game. Like I played the original (not even TTD, just TT) and this is still a very fun game. In a nutshell it's an optimization sandbox since the beginning, with more and more ways to invent and customize solutions for the troubles you make yourself.

About the history of the project you better hear from the horse's mouth: https://www.openttd.org/news/2024/03/06/happy-birthday