My guess would be that it is either:
1) Stuck in "Development Hell". This is quite common; all to often there are various reasons why production has not started. For example, money, "creative differences", actors being unavailable, legal issues, technical problems, etc. Many films are stuck in development hell for years. A good example is "Alita: Battle Angel", which appeared in the cinemas a mere 16 years after being announced.
2) Has been cancelled (another version of Development Hell) but the people involved don't want to announce it for some reason.
https://en.wikipedia.org/wiki/Development_hellOne film that I've been waiting to see for some 26 years since it was first announced is "The Forever War", based on Joe Haldeman's book of the same name. Periodically a new name is attached to the project, then nothing is heard of for a year or two. Ridley Scott has been making it since 2008, and was due to start on it after making "Robin Hood" with Russell Crowe according to this article in The Guardian.
https://www.theguardian.com/film/2008/oct/13/ridley-scott-forever-warhttps://scifi.stackexchange.com/questions/167077/what-if-any-is-the-current-status-of-the-forever-war-movie