thats only because they release new cpu's so much faster. 1155 is a great socket, and should be around through Ivy Bridge, and possibly Haswell release. If AMD released as many cpu's as INTEL, they would have a lot more socket changes too. Intel wants people to buy new chips, but they also need people to buy new mobos so that the companies that feed them money (important thing) can make more money to give them. Hence, new sockets mean new mobos, means more money for companies. its a nasty circle, but that is a reason intel has so much money for research and pushing newer, better products.
I would imagine 1155 socket being around another 2-5 years, depending on what you would call "obsolite".