Because Apple have worked out a fairly good mechanism to prevent the system from not falling to sleep when the lid is closed. Open a Terminal and type: pm

Because Apple have worked out a fairly good mechanism to prevent the system from not falling to sleep when the lid is closed. Open a Terminal and type: pmset -g assertions You will get a list of all processes and which kind of sleep mode they are preventing currently. Having said that, it doesn't work well always. I remember that I had issues with btsync once, which was actively preventing sleep even when the lid was closed. This turned out to be a bug, but it was really annoying. An issue with modern MacBooks is that you cannot see anymore whether the device is sleeping or not as Apple decided to remove the status LED. So it can still run hot in your bag, but it usually does not and good App developers test for this when it applies to their applications.