There's an obvious couple of reasons for the barriers on all the themes that I can think of.
Shortcuts, visual quality, driveability.
Remember when PD made driving on the infield at Ascari and Willow Springs like driving through treacle? Same motivation here, I'd bet; note that PD eventually put those infields back to normal and now rely on the shortcut detection instead, which we know is defined per track. That, combined with the fact those infields existed in the first place, implies that PD are not against free track limits, only shortcuts.
Adequately detecting and proportionately penalising shortcuts on the "track path editor" tracks is tricky compared with a fixed circuit. Maybe PD didn't think that development work was worth the considerable effort, and / or they thought that the "simple solution" (detecting off-track and penalising universally, e.g. treacle) was perhaps worse in some way.
The visual quality aspect is for the distant scenery. That tree or house in the distance looks OK from the track, but if you could break out of the track limits and investigate it, it wouldn't look so great. The underlying terrain texture is also low resolution, and would look terrible when being driven on. Queue hyperbole (but really, who cares?)
The driveability aspect is to do with the underlying terrain; it's low resolution, which would look and feel weird, possibly undriveable, plus it needs all the scenery
and trackside decoration to have collision defined. This is extra work that PD perhaps decided they'd rather spend on FIA sanctioned circuits... perhaps, but probably not (tough). Regardless, they still had their own priorities when dividing content creation effort.
Why are the "barriers" so close on Death Valley? Because, I suspect, the decoration is defined as the barrier on all themes, and the decoration is much closer to the track at Death Valley, by (aesthetic) design. To push the barriers back on this one theme over the others would mean to give the decoration complex collision geometry (simple boxes would be infuriating here), something which wasn't necessary on the ones lined with Armco. Either that or just move the decoration back and lose the aesthetic effect (that might well be something PD could investigate). I hope PD decide the track files can be made larger, to store extra info like track width, barrier distance etc. per section without sacrificing length or the 20 track upload limit. They could use the moon mission barriers for the extents of the terrain, but with a kind of damped springiness instead of a hard stop or annoying reset.
If it were me, I'd have not cared about the low res background, I'd have bounded the scenery items with a simple box for collision and not cared about their visual fidelity, I'd have used a simple (crude), but optional, universal shortcut detection method, I'd have made the barriers an optional part of the decoration step (no barriers, no decoration, though). I'd have used the resources to the detriment of other content, too. But then I wouldn't care about the people complaining about these things, either.
