The answer is pretty obvious in the case of the J-Hack Pure XL; there isn't any other way to fit a compact GPU and an SFX PSU in to 6.5L. They could increase case width to ~240mm and then give people the option of installing a GPU or a PSU at the bottom of the case. Or they could increase case depth to ~256mm and put the SFX PSU next to the RAM, retaining GPU compatibility. But these options increase case volume to 7.15L and 8.7L respectively.
As for the SG05, it hails from a different era. Yeah, there's space to put the PSU at the front. But doing so removes hard drive and optical drive support in favour of better CPU cooling options. Back in 2009, that wouldn't have seemed like a good compromise, firstly because these drives were still critically important to most, and secondly because there were issues limiting the thermal demands of the CPU anyway (e.g. PSU capacities, poor power delivery on ITX boards). If Silverstone ever redesign it, I would be surprised if they didn't move the PSU to the front, as that would be a lot more in keeping with how the market is now.