Because most of the work you're proposing to have inmates do is currently being done, if done at all, by unionized public employees.   I can still hear the late Al Baffone of Laborers' Local 71 howling at us about how our use of "convict labor" was putting his members out of work because we were using inmates to help cook and serve food in the prisons.   And we'd rather our roadsides be scraggly, brushy messes or mass beds of dandelions than put some inmates out with a sling blade and make the State look like somebody cares.