Especially central europe has often very small atc sectors and therefore an incredible number of frequency changes. It is very easy to get loss comm there and passing through two or more sectors until you realize it. With all the crap on 121.5, mentioned above plus other legit but distracting stuff like practice pans (which can be heard well outside UK airspace), frequency tests from every atc unit and so on, it is usually set to a very low volume, otherwise it will lead to problems listening on the primary frequency. Listening and talking on the radio is after all just a part of our job, flying and managing our aircraft and crew has to be done as well. And SOPs with a lot unnecessary callouts, like for example the airbus standard SOPs, can interfere with radio calls as well.