I think you're right that it's a bit weird. It's a legacy thing, though, from back when the kernel needed to be pretty small and 13K meant something. On the other hand, GENERIC does include stuff like "options AHC_REG_PRETTY_PRINT" which, if the comment is to be believed, adds an order of magnitude more to the kernel size than "options SC_PIXEL_MODE".
Also, as the non-technical user-base has grown, so has the discouragement towards custom kernel building (of course, a few months on the freebsd-questions@ list will teach you why).
At this point I'm merely rambling, but I don't think my experience is a good indicator of How To Do Things: I run custom kernels, when something breaks I tend to blame myself (and I've only been wrong in that maybe four times in ten years), & I have pretty good problem solving skills, so google and the mailing list archive solve nearly everything I've had problems with. The noise level on the mailing lists would be unbelievable if everyone fired off angry emails every time they broke something with a bone-headed entry in make.conf, the small number of users who do that right now is pretty noisy as it is.