As title states, why is this? Is it simply just to raise the price of the case or is there some other reason for it?
Sounds strange to me as well. I always had to buy those things separately, which I think is the best option (unless there is a good case with a good PSU being cheaper than buying each separately).
Cheaper cases always used to come with a PSU, just like lower end CPUs come with a heatsink. If you need to buy 20 PCs for a company or a school, you don't nitpick with fanless PSUs or stupidly high wattages, you just need something that works for three years, acceptably well (then they will replaced anyway because fiscal reasons). Cases w/o a PSU are meant for enthusiast who like to build their own personal machine and look at the quality (or the perceived quality) the color and whatever. Say when I bought my Seasonic X400 PSU (a superb fanless unit, for the time I bought it, and it still has few real competitors today) its official street price was 149€, in those days we used to sell complete minitower cases for 16€ (to be clear, with the PSU included). Compare the prices above and spot the differences...