They are achievable, by someone who plays the game for a year...
So an achievement like "Play for 200 years" would be totally fine too? It is achievable after all. There's just no human capable of doing it.
It's just not realistic. You'll probably won't even find someone who achieves "One Month" with just playing.
Since there is no benefit of having the achievement, why does it bother you so much?
Some people just like to complete achievements. Preferably all achievements a game has to offer.
But games like this just ruin the fun part with completely unreasonable achievements.
Achievements are supposed to be fun and/or challenging but this one is neither.
Other examples apart from Garry's Mod are:
Darksiders
"Ride for 100 miles" while playing through the game you didn't even ride 20 so you ended up riding aimlessly for hours.
Mafia 2
"Drive a total of 1,000 miles in vehicles in "Jimmy's Vendetta."
You barely drove 120 in the entire campaign so you had to drive for about 10 hours on the highway.
If you "achieved" it by running the game in the background, that isn't really something you can be proud of, so whats the point?
It's the only way to do it so I guess it's something to be proud of. The first one to achieve the Garry's Mod "1 year" achievement got basically celebrated.
We are completely drifting away from my question by the way.