You likely answered the question when you said you only run for exercise purposes.
This is supposed to be a running message board, and many of the people on here run, and they run a lot. Many run for more than exercise.
They run as much as possible during the week, given the limited time for this due to school or work obligations. They might be getting up early to squeeze in their run before work or school, or even running twice a day. Adding all that running on top of regular life takes time and energy.
On the weekends, they have time to run more, longer, race, etc. All that extra and/or focused running takes more energy. Instead of going out trying to socialize, dance, drink, let loose, etc., these runners are either resting up for more running the next day or recovering from it.
They also may be recovering from the busy week, and gladly staying in and resting/recovering Friday and Saturday nights.