Why does the rear brake bowden have no housing below the frame of the bike? I observed that the bowden of the rear brake calipers runs &quot;naked&quot; below the bike frame. (Also the gear shifting cables, but let's focus on brake cables for this question.)
(I.e. the section between the lever and the frame is covered with housing, then, below the frame, no housing, and then the section connecting the frame and the calipers, again has housing.)
The question is: what is the reason for this? And more importantly, if I replace the bowden for the rear brake, would it be a good idea to add housing below the frame as well (to protect the whole bowden from dirt and water, rather than leave that naked section)? Of course the housing would have to be interrupted at the suspension points, leaving a little bit of bowden &quot;naked&quot;, but at least most of it would be protected?
I tried to come up with some reasons, for why that part is naked:

keep costst down (but that extra half meter of housing would not matter much, would it?)

physically not being possible to add housing, because of how the bowden is suspended: the housing for sure would not fit through the suspension points; however... can't you just cut the housing at that point? Sure a very small part of the bowden will still remain &quot;naked&quot;, but maybe even that would be better than having the whole wire outside? (Or even better, couldn't they have just made the suspension points huge enough to fit the housing in the first place?)

if there was a continuous housing (like I proposed above, with the slightly huger suspension points), maybe the rear brake would not work, because the the wire would be too long, so it would easily bend, instead of the bowden sliding inside it: this seems plausible, but even in this case: wouldn't I be better off, by adding housing (albeit interrupted) below the frame? (and still: couldn't there be a mechanism at the suspension points, which presses the housing together just enough to keep it steady, and at the same time allow the bowden inside of it to slide?)

the interrupted housing would be worse than no housing at all, because it would allow dirt and water to enter, but it would collect, and be more difficult to remove: while this seems indeed plausible, don't we have the same problem where the housed section of the calipers and the levers starts?
You do not really need the housing functionally, the bowden runs straight between two fixed points. There is nothing that could compress or deform. It just works well as is. You only could add it as a mechanical protection but it is not customary and could add some additional unnecessary friction.