Part of it is laziness and callousness to be sure. Part of it is municipalities that don't have a devoted animal shelter. (And there are more than you'd think.) And part of it, assuming the people know about it, may be the kill rate. Memphis Animal Services at one time was infamous among rescue groups nationwide for its kill rate. Too much inflow; not enough output of adoptions. They're working at it and slowly getting better, but (dependent in part on the season) owner surrenders still largely die soon after they're brought through the doors.