I have no idea why people do this either. One time I accidentally started a round of plane landing applause when I clapped sarcastically to point out the stupidity of doing the same.
Near the end of our flight back to New York, I leaned over to my mother and remembered that when I was younger, people used to clap when the plane landed. ‘What a stupid custom,’ I told her. ‘Did these idiots get on the plane, then realize maybe the pilot didn’t know how to land it?’ I railed, quietly, in this fashion for a minute or two and concluded by saying ‘At least I haven’t heard it since September 11th. I guess people decided the possibility that the pilot actually couldn’t land was a little too real and not too funny.’
A few minutes later, the plane landed on US soil. I sarcastically clapped my hands a few times. And then it happened. A few passengers way in front of me started clapping, and a moment later, the whole plane burst into applause.