I had that happen to me once, last year.

From what I read on the internet at the time, if Steam gets shut down without properly updating itself while closing the game's files, the information that tells Steam where the game is located can get corrupted, or something like that, resulting in Steam not recognizing that the game is already installed.

If you search for "Steam says game not installed" or similar phrases, you can find posts about how to try to correct it without having to reinstall the game all over again. Following are a few discussions about this, but I can't vouch for any of them:

https://support.steampowered.com/kb_article.php?ref=8379-RYIP-2998

https://steamcommunity.com/discussions/forum/1/684839199046901613/

https://www.pcerror-fix.com/fix-steam-not-recognizing-installed-games