For me its a matter of availability, & time. I admit to mostly RTR loco's even the DJH kits are built for me, although many are modified by me as well as weathered, with some being modified for me by the builder with my input of what I would like in order to enhance the model. Older RTR R/S are modified or in the process of modifying.
I also have a lot of kits, which are in both the to do box or partially built, same with some kits of infrastructure that have been modified. Although most of my buildings are scratch built with some almost complete with only lighting & decaling to finish them off. I have thoroughly enjoyed building them as against some kits & even modifying some of them as well.
The aspect of the hobby for me is to enjoy it, & that includes each of the types, RTR, Kits & scratch building. The one advantage in having RTR locos & R/S is that when one gets tired of building, its nice to be able to sit back with my radio cab & run the RTR trains that primarily give no problems, except on some areas of track which likes to have loco's momentarily stall at, generally though few stall at the same spot but at different areas which having DCC with Stay alive decoders has resolved.
So, there really is a place for all types in the hobby, I just hope I live long enough to get close to finishing it all.