They don't do it on all instruments.
AFAIK, Fender only does this on instruments that are replicas of vintage instruments. And if they didn't make them that way, they wouldn't be accurate replicas. Now there are some very sound reasons for the truss rod nut being at the heel, mostly having to do with how well it works and it affecting more of the neck. Plus, most of the instruments with a truss rod access at the heel that I've worked on (I managed a guitar store from 1977-1988 and touched a LOT of old Fenders), you don't have to do all that much to get to the truss rod nut.
I have three instruments now which have the old-style Fender truss-rod. My Vintage Series '62 P bass doesn't need anything special at all. The cut-out in the pickguard is scarred, but that's meaningless as I've played the bass since 1983 and she's got some other scars too. The Classic Series '60s Strat requires me to remove the pickguard assembly (and hence the strings because Start PUPs are mounted on the 'guard). That's a PIA but the guitar sounds and feels perfect so it's a small price. Plus I don't "tweak" stuff much at gigs- I do that before the gig, which I realize may not always be practical. The other instrument is a Sting P Bass. I have to take the pickguard off that bass, but it's not a big hassle. Just remove the screws, slide the guard out of the way, and the truss rod nut is easy to get to.
If you don't want that adjustment, then buy a Fender-style bass without it. I believe the new US production (again, except for replicas of vintage stuff) all has easy truss rod access. Or get a Lakland- they all have easy access at the heel.
John