It's not a 'lack of time' why people don't exercise, they don't make time to do it. I totally understand why: it's so much easier and convenient to not do it, and for a very long time, you don't notice it all that much if you don't if you fail to do any exercise; it's only afters a long time of being sedentary and then having to do something slightly intensive you notice that all your strength and stamina is gone.
It's not lack of time; it's that people do not actively feel how badly their bodies deteriorate when they do not exercise, since it goes so slowly, and they do not do any exercise, so they also never really find out until it has become a real problem. If people would feel how bad it was not to exercise, they would be much more accommodating in their immensely busy schedules and actually take time out of their day to do so.
Once you cross certain threshold in fitness, and keep that level for some time, it becomes painfully obvious when you stop and almost immediately start losing all that hard gained strength, stamina, flexibility etc.
Relatively high fitness level is such a great feeling from oneself, it also positively affects your mind and mood. Every time I stop for whatever reason (ie travelling, sickness) I hate it, and also remember how hard it is to get back.
It's not lack of time; it's that people do not actively feel how badly their bodies deteriorate when they do not exercise, since it goes so slowly, and they do not do any exercise, so they also never really find out until it has become a real problem. If people would feel how bad it was not to exercise, they would be much more accommodating in their immensely busy schedules and actually take time out of their day to do so.