I couldn't agree more with your frustration. All too often, I see riders compromising their safety and performance by using subpar components and poor maintenance practices. When it comes to brake cables on a gravel bike, there's no room for compromise.
Cheap, low-end components may save you a few bucks in the short term, but they'll end up costing you more in the long run. Not only will they compromise your braking system, but they'll also wear out faster and require more frequent replacements.
And as for those ancient compressionless brake housings, they're a recipe for disaster off-road. They may have been fine for road bikes back in the day, but they simply can't handle the rigors of gravel riding. They're prone to kinking and binding, which can lead to catastrophic brake failure when you need them most.
As for cable routing, it's a wonder that some riders even bother. Poorly routed cables can cause all sorts of problems, from increased cable friction to premature wear and tear. It's not just about aesthetics - proper cable routing is essential for optimal performance and safety.
So, to answer your question, the most egregious offense when it comes to gravel bike cable maintenance is compromising on quality and neglecting proper maintenance practices. Don't be that rider - invest in high-quality components and take the time to properly maintain your bike. Your safety and performance depend on it.