Sometimes once just isn’t enough...
Bands have made the encore just another part of the set, its lost its meaning and true appeal as most of the time you know that whether you clap or not the band are coming back onstage - normally to play the singles they 'saved' from the main gig for the encore.
Why? because they knew they were going to come back - which completely goes against what an encore is, it is like leaving a tip before you have eaten a meal.
So why clap at the end of a gig if you already know the band are coming back onstage regardless - you might as well save your energy and try and get them out for a second encore that they have not planned for.
If enough of you cheer, clap, chant and scream for a second encore how can they not?