Sorry if people not wanting one has somehow offended you!
Whereas I think they are pretty good ideas, and would be happy if they were built in, I have a lot of basses, I bought 10 alkaline 9v batteries for a tenner, and i have never had an issue of a battery running out on a gig (one that went before a gig but it was a new bass and I didn't realise it was wired wrong). I don't tend to change them that often, but I tend to play with the ones I am gigging.
I do always have spare batteries though, and as almost all but one basses have a battery compartment that doesn't need a screwdriver, it is a 30 second job to change it, so it seems like an overkill to retrofit.
So not a problem of not being bothered, more a question of the cost and inconvenience of putting one in is vastly more than the inconvenience of not having one.