A few reasons:
-I manage a global team. I have team members in India, Japan, Korea, London, Singapore, and near me in the Bay Area. Getting calls/meeting together requires after hours for most people involved. I also travel to meet with them. I leave on Sunday, for example. Which will put me in Tokyo on Monday night (time zones). So, there's one day of my weekend gone, and i still miss a working day in the office. I will take the Friday evening flight home, putting me in San Francisco on Friday afternoon. I will be jet lagged. My weekend will suck, as I try to adjust. I will also be way behind on my actual job, due to traveling, & will need to figure out how to catch up.
-My job is in tech & retail. Our busiest times of the year are when people have time off. All around the world. Mostly these match with my own holidays & weekends. That's when we have something big that needs to be executed. We plan ahead, but technology fails. One of our biggest production tools failed last night. Not my problem, but my peer was on a call until midnight.