Phone calls, people talking around you, movement everywhere, unwanted taps on your shoulder and a lot more.
Unless your job provides you with a closed office where you can actually focus on, it's pretty much the same as having kids around.
When I worked in an office I remember the most productive hour I had was right after I got there, because I arrived ~1h early (due to commuting BS) and was basically alone.
Once teammates arrive, it's chaos. No matter how good your headphones are, something will interrupt you at random times just like your kids. Except if you do have an office in your house, you can actually close the door and have uninterrupted time, even if it's in small chunks.