A well written software shouldn't do this. But I also never had such problem, neither with Steam nor with any other kind of thing. I would demand either a fix from Steam or a refund. They are effectively stealing your time (and money).

Funny, I've had this same problem for a while now, mostly with Source Engine games (Half-Life, Dark Messiah) but also with a couple of others such as Binding of Isaac. I suspect in the case of the Source Engine games, it's related to the updates of the Source Engine runtime.

Never really bothered me like that though and I just chalked it up to one of Steam's many glitches. Either I reinstalled from my backup when I had a slower connection or I redownloaded.
Spend a little more time on these forums than your reps would suggest and you'll learn that this forum is for more than just GOG stuff.

Aye I'm aware of that. I just thought that Steam places would be better for Steam support. But I've been told that the steam forums aren't very good so GOG members use this forum for Steam support instead. Which makes sense.

When I'm not actively installing/updating a game or the Steam client itself I lock Steam in offline mode with the steam.cfg method. This is great for setting up a known working state; games that are working when you set the offline mode stay working (Steam can't see any pending updates).

I'm not sure how well Valve's multiplayer games work in offline mode but anything that doesn't use Valve's multiplayer system will be fully functional.

Another good idea is to test your HDD

I checked Steam forums and there are plenty threads about this issue and nobody found solution yet. Steam support don't know how to fix it either.

I can't check local files because such option is unavailable for games that are not installed and when this problem occurs, Steam consider those games uninstalled even tho files are on the disc.

My hard drive is fine. I bought a new one recently and this problem still occur just as on old one. I don't have problem with anything else, all programs, files, OS, games and other clients (Desura, Origin, Uplay) work perfectly fine.

Some months ago I managed to get in touch with a real, live, competent Valve rep via email who wasn't just re-pasting stuff from their FAQ. Your problem sounds a little like the one I had, so I'll offer the answer I was given:

A good while ago, Steam began to change the way their games are stored on your computer. It's considered an optimization, and the client generally can compress your files and download what's necessary without you being very aware of it. There are some cases where the Steam client can't, and those cases seem to be less about what you're running as a user, and more about how the various hops getting traffic to you manage that traffic.

The answer made sense given where I was at the time and how awful my internet connectivity was. It's strange to me that near two years after they started their file format change that games are still throwing up all over everything, but it's somewhat plausible. The fix for me was to just suck it up and remove and reinstall the games I really wanted to play. But if I had it to do over again, I'd back up the game data locally, remove the game from the steam client, restart the computer (and also thus the Steam client), and recover the game data in the hopes of not having to redownload the whole mess. If you've got good broadband, though, you could set it to run overnight and it wouldn't be a problem.

They are rolling out a new feature called steampipe , This might have something to do with your games
https://support.steampowered.com/kb_article.php?ref=7388-QPFN-2491

Absolutely Licurg! This is what i planning to do. But then how we can play the Steam-only games?

You get them DRM-free from thepiratebay or similar sites. Ideally, every time you pirate a game, send the publisher an e-mail and tell him "I wanted to buy your game, but because of your idiot DRM I pirated it". If we'd all do that, then we'd finally get rid of this DRM situation...

Have you ever done this? Are they replying? =)

Not yet. But that's just because I don't play steam-only games, because my PC can't run any of them :/ But first thing when I get a new PC, I'm e-mailing Sega and telling them that I pirated Shogun 2 :D

Lol!

And if you feel guilty and want to go the legitimate route, you can purchase a broken game on Steam and then liberate it from and fix it with a nocd patch.

That sounds very risky there. I think it would be better to do what I have seen others do, thats is buy the game and then download a pirate copy. Then you can email them. Other wise they may sue you, ofcourse they could sue you either way but they can't prove damages if you bought it first.

I have read a good number of forum posts of people doing this for all Assassin's Creed games post the first one.