99-year ground leases are de facto sales. They're just structured as "leases" for legal, financing, and tax reasons. UMass Boston doesn't, for example, actually intend to get the land under the future "Dorchester Bay City" development back in 2118 and incorporate it into their campus. That land is gone from UMass's perspective.
And even if 99-year leases were indeed just normal leases that terminated and reverted to the owner, that would still tie up the land for, well, 99 years. MGH may not have public plans for land behind North Station today, but nobody thinks they'll actually keep it as a parking lot until 2120.
Maybe they don't plan on using it for anything more productive than parking in the next, say, 15-20 years, but that's well short of the life of anything else that would be worth building on the land as an alternative.