The static set lists are due to a combo of overly choreographed production and U2 admittedly being mediocre musicians who can't churn out any random song on a dime. Same goes for their general showmanship. Bono's entire performance, movements and mannerisms, are rehearsed. There are obviously multiple contributing factors here.
I disagree with all of this.
Are U2 the best musicians? While we aren't getting an artist like Springsteen and his band mates up there, I have never been disappointed at a U2 show. Edge's unique style puts him up there. And Bono's vocals also have him consistently considered one of the best vocalists ever. Adam and Larry have separately produced some brilliant bass lines or beats (this was showcased early on with NYD and SBS).
Technology does play a role. The electronics and videos are tied into a playlist. If U2 randomly does a deviation, that will cause problems or look sloppy. That said, I have enjoyed some rare treats, like the sudden performance of "Fast Cars" during the "Vertigo" tour.
While U2 aren't just a jukebox capable of playing all of their songs upon request, I can't blame them. Whenever I give a presentation, I rehearse. Two or three years later, I can't just give that same presentation without rehearsing again. Same is with U2.
Maybe some bands can just jam out, but I assure you, most of those artists that appear to "jam" are well rehearsed. Plus, U2 have "jammed" in their own way, with their extended versions of many songs - such as SBS, "Bad", WOWY, MW, "One", "Elevation", BD, etc.
I realize people like spontaneity, but that's not who this band is. U2 may have mixed things up during the Love Town tour, but really, it was a select group of songs that they simply altered each night.