If you enter an open plan office and see a bunch of people sitting with headphones on, it should be immediately clear to you that most of them are suffering from noise pollution. The headphones are usually a coping mechanism to keep out the noise, not primarily to enjoy some music.
The problem with open plan offices is usually the fact, that it gets incredibly noisy. increased noise levels lead to being distracted, which leads to exhaustion, which leads to lower productivity, which in some cases causes mental problems like burnout syndrome etc...
this might be not the case for every type of person, but i had the "pleasure" of experiencing the upper situation personally and a few times in other ex-coworkers who just couldn't cope with the constant noise.