Why is there so much weekend work?

Now that the has been using Fastrack, one would think that there would be less work on weekends, but looking at the updates, it shows that every line in the system has weekend work, save the SIR, the ,  and  and the Shuttle.  That begs the question of why is there still so much weekend work if they're also doing Fastrack, and rather frequently I would add?  I know that they claim that ridership is lower in the summertime, but I find that hard to believe, given the amount of people packing the streets in the city that I see.