The kit's based on an S-series rather than an RG (though I don't know how closely the dimensions match up with the original - I think it's a bit thicker). But on the originals, there simply isn't enough depth of body on the edges to fit a jack socket on the edge. There might possibly be enough depth to just about fit a barrel jack, but there would be almost no wood on the top or bottom of it, so catching the jack plug on something would very likely result in the barrel jack being torn out of the body. Which isn't a good look.
So that's why the S-series has the top-mounted jack, and as the kit is based on that, then that's why the kit has one as well.
The kit must be a bit thicker-bodied than the S, as you don't need the 5-way switch being mounted on a surface-mounted plate in order to fit it in, so it's possible the socket could have been fitted on the edge, but then it would lose the S-style look.