A lot of that real estate actually isn't owned by the post office, but leased. Hundred year lease, but still a lease.
Post Office is actually one of the few things the Federal Government is doing that is actually constitutional. We'll have to add removing it to the “article V” constitution convention, eh?
https://about.usps.com/publications/pub100/pub100_005.htm
The Constitution and the Post Office
In June 1788, the ninth state ratified the Constitution, which gave Congress the power “To establish Post Offices and post Roads” in Article I, Section 8. A year later, the Act of September 22, 1789 (1 Stat. 70), continued the Post Office and made the Postmaster General subject to the direction of the President. Four days later, President Washington appointed Samuel Osgood as the first Postmaster General under the Constitution. A population of almost four million was served by 75 Post Offices and about 2,400 miles of post roads.
The Post Office received two one-year extensions by the Acts of August 4, 1790 (1 Stat. 178), and March 3, 1791 (1 Stat. 218). The Act of February 20, 1792 (1 Stat. 232), continued the Post Office for another two years and formally admitted newspapers to the mails, gave Congress the power to establish post routes, and prohibited postal officials from opening letters. Later legislation enlarged the duties of the Post Office, strengthened and unified its organization, and provided rules for its development. The Act of May 8, 1794 (1 Stat. 354), continued the Post Office indefinitely.
The Post Office moved from Philadelphia in 1800 when Washington, D.C., became the seat of government. Two horse-drawn wagons carried all postal records, furniture, and supplies.