Because the federal government has no power to hold onto a state's land without using it for some constitutionally-authorized purpose. So ultimately it's an arg

Because the federal government has no power to hold onto a state's land without using it for some constitutionally-authorized purpose. So ultimately it's an argument based on state sovereignty. But aside from that "As the federal government itself recognizes, state and local governments suffer 'losses in property taxes due to the existence of nontaxable Federal lands within their boundaries,' owing to their 'inability … to collect property taxes on federally owned land.'”