Cost.
Several of them are hosting cell and radio services so that "pays the rent" on the location.
Dismantling and scrapping such a tower would be enormously expensive and bring no value to AT&T shareholders.
If they're in total disrepair, I'm sure a case can be made for safety in an urban area. The two towers closest to me are in rural areas with several other radio service tenants where maintenance is a covered cost.
Legacy AT&T owns almost none of them anymore, what's left are owned by the RBOC (often the other AT&T) and as you note, have a prohibitive removal cost and are in urban centers.
Dismantling and scrapping such a tower would be enormously expensive and bring no value to AT&T shareholders.
If they're in total disrepair, I'm sure a case can be made for safety in an urban area. The two towers closest to me are in rural areas with several other radio service tenants where maintenance is a covered cost.