Why are there many apartment blocks on sale

Tell them.

maybe at the higher end of the sector, but I doubt banks and insurance companies are putting up the ever increasing flat/ apartments /rentals - whatever they are called in the mid and low income areas, akina kayole, kasarani and githurai type of areas. Of course its hard to ascertain some of the rumors, but I would not be surprised if a good number of owners of such buildings are politicians.