Many cities have plenty of existing buildings that could be repurposed and many cities real estate prices are extremely high due to the land already being built out. Very few if any government budgets can afford (nor should they) to place homeless in ultra-expensive markets; it would be inefficient to do this.
Homeless may need to be placed in outlying areas (outside even the jurisdiction of where the homeless originated) where real estate is cheaper. Coordinating this though may be difficult since many homeless bring crime and drugs with them.