well for me... I'm table saw rich and cnc poor... but I know a guy who could do one up for me. further, there are not a lot of options for multiscale slotting templates that aren't uber-expensive so I would think if someone were to make these they'd be comparatively inexpensive and as such: popular among folks who don't have cnc... maybe.
in addition (and please comment on this) as I understand it, slotting a board on cnc is a lot of work... it requires delicate bits that break... and requires different programming for each radius, no? It can certainly be done... but might not be "worth it" to some if you consider how fast one could achieve this on a table saw. perhaps not... i dunno, pure speculation... you tell me.