They aren't. I still have some 2-years-old Steam keys.
If you still have them (non-activated), how do you know they still work?
As far as I'm aware, the only way to know if a Steam key works no matter how old it is, is to try to redeem it. Having said that, I have Steam keys that are over 2 years old and I have recently redeemed some with no problems. I have never encountered a case in which I bought a Steam key either from a legitimate storefront nor a bundle in which the key later expired before I redeemed it, and I'm unaware of any incidence in which that has ever happened for anyone other than perhaps time-limited promotion codes for freebie games or similar such as the extra bonus keys that came along with Insurgency in a recent bundle, where you got Insurgency with the bundle with no time limit on it, but it magically came with 3 extra bonus keys for your friends that did have an expiry date on them.
Likewise, GOG has given out promo keys for free games in the past, some of which had an expiry date on them. The Fallout 1/2/Tactics freebie giveaway promo before the games left the store catalogue is an example of freebie game codes expiring on GOG. This same type of promo might possibly occur on Steam as well, but that would be no different from GOG in that sense.
Other than a special case of a limited time promotion for a free game code like this however, I have never seen nor heard of someone actually purchasing a game from a legitimate retailer or bundle provider, and having their game codes expire after a certain date, so I believe any fear someone might have of this ever happening is unfounded.
If anyone has actually ever bought a game from a legitimate retailer, paid money for it and then later found their game codes expired, and it has really happened, I'd love to hear about it though, preferrably with links to online articles of it being a general phenomenon that affected many people and not a one off store accident/glitch/fluke. :)