Why does Steam require re-downloading already downloaded games?

[Super Buni]

Hello. I am new to this community. I wanted to ask if you know why Steam requires me to re-download games to play them again.

I have downloaded games from 800 megabytes to 30 gigabytes and in various situations, by chance, several of these games have had to be downloaded again to be able to play them. Is this some kind of bug in the platform or is this how the service usually operates?

I appreciate any guidance.

[Jerry]

Unless we are including the possibility of updates, it is definitely a bug, but rather on your end. Somehow Steam fails to recognise the installed files. A possible cause could be, that they are on an external drive, that is not attached every time when you launch Steam. If the files are missing, Steam will note, that the game is not installed.
There could also be some issue with the acf files, that are connected to the individual games. Or maybe, your installation is in some way broken.

[Super Buni]

I wish there was some way to monitor the hard drive. some games I install on an SSD, and others on a traditional hard drive.

[Bee🐝]

Official FAQ:

Installed games are appearing as uninstalled: https://help.steampowered.com/en/faqs/view/4578-18A7-C819-8620
Read the WHOLE page carefully first and then get back to us.

[Super Buni]

I have installed Steam first on a common hard drive and then on an SSD. maybe I should uninstall steam completely, delete the games from the hard drive and start reinstalling everything from scratch on the regular hard drive?

I think if the games stop being recognized by my computer again, I'll have to do a deep clean.

Thanks for the advice.