Are they now. Because you say they are?
By your logic, an Act of congress can legally create slavery or ban guns, etc. regardless of what the Constitution says.
No it can't because the 13th and 2nd Amendments say they can't. But nothing explicitly states that the federal government can't own lands within the states. The very fact that we've created 37 states since the ratification of the Constitution shows that.
US Constitution Article 8 section 17 specifically limits what land the federal government can own within a State (as contrasted with a non-State territory) and how it has to go about it.