How is one dignified and the other not?

The efforts of those road workers are effectively meaningless. Worthless. Moot. Their efforts would be better expended elsewhere - service jobs, creative occupations, the like - yet here they are, digging ditches for subsistence wages (at best) instead of doing something properly meaningful with their time on Earth and their energy.

The idea in places like India that manual labor takes precedent is a symptom - I'd argue - of exposure to the lingering remnants of a very long-lived caste system; instead of pushing these laborers into more useful fields, there's a preference to relegate them to menial, worthless work under the guise of "public service" or "charity" in order to reinforce that system, whether deliberately or perhaps unconsciously. I can assure you that, of the many reasons to emphasize such manual labor, "dignity" of the laborers is not one of them in this case.