SSA's field office structure has gotten some political attention recently, as they've closed some offices and have a study which recommends further changes Their unions, and AARP, are up in arms. They still have 1250 field offices.
It seems to be a similar problem to USDA and its field offices. The logic of computers and internet and declining populations in some ares leads to office closures, while the logic of serving the public and not further undermining the economics of localities lead to preserving the status quo.
SSA may have a better case for consolidation and online service than USDA: their programs don't need local data. Presumably if you're disabled for work in Deaf Smith County, TX you're also disabled in Fairfax County, VA, and vice versa. If I need to apply for SS benefits it shouldn't matter if I'm in Florida on a vacation or not whereas USDA programs are tied to geography.