Developers will do this anyway if the offices are empty, why not use that money for a government program to guarantee down payments of first time home buyers?

Because that doesn't do anything but provide guaranteed cash to existing property owners at the expense of people trying to stop renting. Until the supply side issue is addressed, homeownership will continue to be out of reach for most. The best case here would be to convert these buildings into condos or whatever the local word for apartments you own instead of rent is, but just rentals us a good second choice.