Have you seen how a roadside mower works? Because a guardrail between the walkway and the road would definitely impede that. And then for ada compliance, you'd need to make sure the rails are finished on both sides, and there are sufficient gaps at the crosswalks and curb cuts. And, you'd still need some sort of protection on the ledge side to prevent people from falling down into the bushes.

I mean, yeah, unlimited budget and effort, you could make that walkway much better. You could pave the whole thing and level the ground so there is no ditch and no trees. Plant some gardens for pollinators, and put in a water feature to keep things cool. Build a playground and one of those moving walkway conveyors they have at airports. Ice cream and blowjobs for everyone, while we're at it.

The way it is now protects cars and people from the ditch, and is easy and cheap to maintain. Hardly any sidewalks anywhere have guardrails along the curb to protect pedestrians, because most pedestrians are hit where they cross the street. Even if a car jumped the curb and hit the rail, it's unlikely people would be standing in that exact spot, and how often does that happen anyway?

If you want to improve the walkability shown in the picture, you'd do better putting in more crosswalks, signals, signs, and stops to permit pedestrians to cross the street more safely.